Switches
Second things first: with regard to switches, read the following FAQ. In particular, in the U.K. the devolo and poppbattery operated switches work very well with smartThings.
OK, now as far as repeating.
Zigbee light bulbs
By far the simplest thing for lighting is just to get the Phillips hue bridge, attach your bulbs to that bridge, then use the official SmartThings integration between the hub and the bridge.
Any light attached to the hue bridge will be able to repeat for other lights attached to the hue bridge. It will work automatically, it will work well, and you will have a strong network for your lights. You also then be able to use other third-party app, including IFTTT, with your bridge, which can create additional effects like links or color loops. Again, simple and reliable.
The new Hue personal white bulb costs around £15, in line with most of the other inexpensive smart bulbs.
https://www.amazon.co.uk/Philips-Personal-Wireless-Lighting-Edison/dp/B0152WXI0E/
While it is true that zigbee bulbs attached directly to the smartthings hub will take on the ZHA profile instead of ZLL and will show on a network map (using a third-party mapper, smart things does not supply one) as REPEATERS, the problem is that they just don’t seem to be very reliable repeaters when repeating for other device classes than bulbs.
Some community members have speculated that there’s a Timing issue with the bulbs. Whatever it is, they just miss some messages. So having a ZHA bulb may improve your network, but you should never rely on it as the only ZHA repeater in a zone.
ZHA Repeaters for the UK
The simplest ZHA repeaters for the UK market are the smartthings pocket sockets. But they can get expensive if you don’t have a secondary need for them. Unfortunately, There just aren’t a lot of zigbee home automation devices available in the UK, so after utilizing what comes in the initial kit, I would tend to recommend that UK members go with Z wave and compatible Wi-Fi devices instead.
So the short answer to your original question: to keep things simple and reliable, let the Hue bridge create its own mini network with the bulbs attached to it. Each bridge can support up to 50 bulbs. These bulbs will repeat for each other but not for anything else.
For the rest of your Zigbee devices, use the SmartThings pocket socket as the ZHA repeater for each zone. If you can find a less expensive ZHA pocket socket in the UK, use it, but I haven’t yet heard of one.
Other than that, for the UK market your life will be much easier if you stick to Zwave and smartthings-compatible Wi-Fi devices. I understand that’s the opposite of what you hoped for, but I think it’s just the reality of what’s available in the UK.
The following is the official list of UK-compatible devices: