would it be legal for me and my mates to rent out a flat at 16?

No. Simple as. Unfortunately, at 16 you aren’t legally old enough to live on your own without parental or guardian guidance, you can enter the lottery though! A flat in St.Albans would be expensive, even a studio flat. The more rooms you have, the more expensive and a two bedroom flat costs about £250k alone and if your thinking of renting, the living costs are about £1000 week, perhaps more with a few of you (bearing in mind that most of that £1000 is rent money!). St.Albans is good for schools (or so I hear) and as parents will want to have their children study there, getting a cheap house is going to be VERY hard, especially as tennents must agree to let you live there first and they are more likely to allow families than British (I assume British?) teenagers, even if you are 18! So, no, you can’t and it’s very expensive….get a dog….much easier