Hi,
I was in Laos on the road that heads to Lantouy in September (I didn't actually go to the border itself), and at least one of local told me that the border was open to foreigners. However, foreigners are very rare in that part of Laos, and I've never met anyone international who's actually crossed it. It may be open to foreigners, but if you attempt it, you'll be a bit of a guinea pig. I'm also not sure of whether visas are available here; if you want to attempt it, you'll probably increase your chances by getting a Laos visa in advance.
The Boten crossing is the main international crossing, and is very straight-forward. I'm also British, and I passed through here in September. I was charged 37 USD for the visa (2 USD more than the price ought to be...). Try and have USD for the visa if you come through here. The main road from Kunming all the way to the border is a fancy expressway that didn't look particularly bike-friendly to my eyes. Presumably some of the older roads linking the south of Yunnan to Kunming still exist, but expect a lot of hills.