
1 - look out for a companies house or VAT number.
2 - Make sure they have a UK landline number (not 0800, 0844, 0845, 0203 etc) such as 0207, 0161, 0113 etc.
Call them.
There should be no strange dial tones and the person on the other end should sound native to the UK.
3 - Be wary of low prices.
4 - Check if they advertise (those who don't rely on shady games and create one-time websites for few customers).
5 - Finally look out for a UK bank account.