I paid for this service convinced I would be ripped off. I was wrong and probably owe them an apology for being difficult.
In any case, it wasn't all smooth sailing. The tray with the IMEI number was wrong in my iPhone (bought in the US - dodgy it seems!). So I paid to unlock a random persons. I had to send them my phone for them to take a look and it they spotted the problem.
Another £50 later it was unlocked and returned to me.
So long story short, you can trust them to unlock your iPhone, you just have to be comfortable with the price.