Just one thing to consider if you have never had fish is that the smaller the tank the faster it goes critical if something goes wrong.
Over feeding, heater failure etc all go nuclear faster the smaller the volume of water.
10 gall is decent for a starter.
For a single betta though people often go smaller
The Fluval flex kit is a really decent bit of kit and probably actually better than a 10 gallon iffy brand.
Really depends how much you want to spend but decent kit will last.
It also looks really nice
They do a larger ones as well
Free delivery and returns on eligible orders. Buy Fluval 15005 Flex 34 Aquarium Set, L, White at Amazon UK.
www.amazon.co.uk