Please please don't buy anything from Sports Direct. It's not a bike shop and if anything goes wrong which it probably will they don't have the necessary expertise to deal with it.
If your want buy from a national chain
Halfords or
Decathlon are much better bets in fact Decathlon have some of the best value bikes out there and I think they let you try them out.
Most bikes over £250 should have decent enough wheels to support you.
Try to get a saddle that suits you even if that means buying a replacement.
Edit: sorry misread your post you're buying used but advice still stands. If you want to dip your toe second hand can throw up a few bargains. Try to buy one that hasn't had much use. Sometimes bikes are bought on a whim then thrown in a shed.
Make sure you get the correct size which will be around a 56cm - 58cm in your case.
Just to translate BSO stands for Bicycle Shaped Object! In other words not a proper bike.