The cheapest seller (currently Onebo, selling at £15) has only a 3.5/5 star rating, only 25% positive review. Some of the buyers complain that the microSD doesn't even have a Sandisk logo. I am not quite sure you want to buy from this seller.
I really recommend clicking the "63 new" line, them scroll down choosing Amazon UK as a seller. It's more expensive, but you're sure that you don't buy a fake card.
Regarding 64GB and class 10, it's totally your choice. 64 GB sure can store lots of stuff, and you can copy (from the computer) to the card quite fast.
In the other hand, 64GB class 10 card is about 3-4 times more expensive than the 32GB class 4 counter part. Not the most cost efficient option. In additional, the phone won't be able to utilize the class 10 bandwidth in most of your daily activities.
Edit: I bought a 32GB class 4 Kingston for HTC Vision, at $30 (around £15) shipped and sold by Amazon.com