I'm not sure about the UK. However, one way you could go, which will ultimately save you money in the long run is to ferment your own probiotic such as water kefir. You can often order the kefir grains over the internet, but be wary because these products may be dead and therefore, unusable. I found a local place in my area that sold me the grains. Once you get a couple batches going, the grains start to replicate. You end up with more grains than you started with. As long as you keep feeding them with Sugar
they'll last forever.
If you don't want to make your own, theralac is a good product, but expensive. I believe one bottle lasts around 10 weeks, though. I also like s. boulardii, but from what I've read, it is transient and does not repopulate the GI tract. Kombucha has s. boulardii and tastes pretty good. I always feel better after drinking it.
You can also make your own sauerkraut. Its simple, but time consuming. Kimchi is another good alternative if you can find it in your area.
Hope this helps.