Quick update for 2019:
At this point, the least expensive plug in both the US and the UK is the IKEA Tradfri zigbee plug. Just a simple on/off device, but works well with SmartThings, is a zigbee repeater, and even repeats well for the Xioami sensors. It will say it needs its gateway, but you can use the SmartThings hub instead. £9 in the UK, $9 in the US, similarly low priced in other markets.
It’s big and only has one socket, but as a zigbee repeater it’s a good inexpensive choice.
Its not that big JD !! Comparing it to other zwave Uk plugs i think its actually middle ish, i have some greenwave zwave sockets and those bad boys are LARGE , wifi plug outlets are comparable to the Tradfri sockets for size
Most compact i have seen is actually the St plug
And i would recommend the tradfri for installation in areas that are hard to reach and or do not require power monitoring
BTW, it is being sold at the IKEA stores in Finland, so I assume it has a local plug type but you can check. Here’s a post from one member who bought it there:
I already purchased couple of z wave plugs which are working great. I could buy couple of those too just to test how they work. They are really cheap!!
Also, as far as repeaters go, Z wave repeat only for Z wave and zigbee repeats only for zigbee. So your Z wave plugs will be good repeaters for your Z wave devices. But the Tradfri plugs are zigbee, so they will work as zigbee repeaters for your zigbee devices. So lots of people will use some of each. 
Has anyone seen a UK power strip that is SmartThings compatible?
I’ve seen this one; https://www.amazon.co.uk/dp/B07KM7N7F4/ref=cm_sw_em_r_mt_dp_U_kvQACbYCAKH89
that says it is SmartThings compataible, but comments say otherwise…
its say it will be soon, its wifi so will be cloud, via a smart app and 3rd party site