Individual zwave bulbs will typically be more expensive than individual zigbee bulbs. Since one Hue bridge can support up to 40 bulbs, if you buy it in a starter kit, it should work fine with SmartThings and if you intend to ever have more than six bulbs total, the Hue solution should end up costing less.
The available solutions at the low cost end will depend on exactly what you were trying to do and how reliable you need it to be. As with many electronics, some of the very least expensive solutions will be less reliable. It’s not a one to one correspondence, but it is a factor.
So, are you looking for smart bulbs, plug-ins, or wall switches? The solutions are different, but all can provide lighting.
Smart Bulbs
A hue white bulb should cost around £15. This is a good reliable bulb and widely available. Sometimes it cost a little less if you get it in a set, typically £25 for a set of two.
Although the hue bulbs can be connected directly to the smart things hub without using the hue bridge, that method is not officially supported and can introduce other problems into your network, so I don’t recommend it.
You can get a starter pack with the current generation for about £49, which means you only paid about £15 for the bridge itself which is why as long as you’re going to have several bulbs, the extra cost will still be within your total budget.
https://www.amazon.co.uk/Philips-Personal-Wireless-Lighting-Starter/dp/B0152WXI18/
There are some other individual bulb brands which will also work with the hue bridge and might save you a few pounds per lamp.
The Innr brand is popular, but will require that you have the hue bridge to get them to work with SmartThings. This is a good GU 10 style.
IKEA has said that their new Tradfri brand is going to work with the hue bridge soon, but last I looked it didn’t yet. Once it does, that again might save you a couple of pounds per lamp if you want to go with those.
But these are all in the same range as the hue white, so I don’t know that you really need to bother with the bargain brands unless you want to start looking at the color changing ones. Then the savings are more significant.
In contrast, the least expensive Z wave White – only bulb will probably cost 35 pounds. So you don’t have to buy a bridge, and if you only need one lamp, then the zwave one will be cheaper. But as soon as you go to two, the zigbee solutions will generally cost less in total.
Plug in Modules
We call these “pocket sockets” in this forum to distinguish them from in wall outlets and micro controllers.
Greenwave makes a good budget line of this which typically run about 20 pounds for one, or as little as 15 pounds when bought in a multi pack. Vesternet carries them at a good price for the UK plug style.
You will not require an additional bridge device with these, they use Zwave. So this is a good simple solution for things which plug-in.
Inline wired devices
We usually call these “micros.” They go in the wall behind the light switch. There are some very nice ones like Fibaro and Aeotec. They have lots of features, are fairly simple to wire, and have full safety certifications. However, they are not going to fit your budget.
There is a Chinese WiFi alternative which some community members use, but they will require cutting and splicing wires and some significant technical expertise to set them up. For example, you’re going to have to flash the firmware. If you don’t already know what that means, I wouldn’t suggest going into these. But they do exist, so I am going to include them for completeness.
The best brand of these, and the only one I think should be considered for safety reasons, it is Sonoff. You can buy these on Amazon. Prices will vary from about 9 pounds to about 20 depending on the features and how much work you have to do yourself.
https://www.amazon.co.uk/Sonoff-Basic-Wireless-Assistant-Required/dp/B06WWNBD3Y/
If you are interested in pursuing this option, talk to the people in the following thread:
Noncertified Devices
There is also a line of inexpensive home automation devices from a very big Chinese company, Xiaomi. These are zigbee, but not certified for the same profile that smartthings uses, and they can be quirky. And I believe twice they have all stopped working for a while after SmartThings firmware update. They are popular because they are cheap, but they are really intended to work with their own hub, and you are going to have to do a lot of hands-on fiddling to get them set up and keep working.
I’ve seen some, and they’re not badly engineered, they’re just not really intended to be working with any other brand’s controllers, so they didn’t bother to get them certified.
They have bulbs, pocket sockets, some switches, and sensors. All very low cost when purchased through GearBest. But community experience tells us you will have more ongoing issues with these then you will with the other brands mentioned so far. So make sure you talk to people in some of the Xiaomi threads before jumping into this brand. It’s definitely not going to be a plug-and-play experience. 
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Conclusion
So those are some of the least expensive options available in the UK.
My personal recommendation would be just to use Hue White smart bulbs with the Hue bridge (£49 for a starter kit of bridge and two lamps) And then eventually maybe add some of the IKEA devices if they turn out to be hue bridge – compatible. And use the TKBhome pocket sockets where you need that kind of device. This approach will be simple, flexible, reliable, and should fit the budget that you gave. 
If you really want to save every penny, you can start looking into Sonoff or even Xiaomi. But both of those are going to require a lot more hands-on work to set up and keep running.