Yes, it's all legal here in the UK. Exchanges have to register and traders have to pay capital gains tax.
Lol, mate you don't have to pay capital gains taxes unless you make quite a lot of money. The allowance is £11,750 this year so if you make any more than that you just have to pay 20% from that sum (might be more than 20% if you make way more). So if you make 12k this year, you just have to pay 20% from 12,000-11,750=£250. Thus it's only 40 quid in taxes.