Has anyone ever accepted an offer of employment and then changed their mind and "cancelled it" in the UK?
I have an offer starting 6 months from now and I am worried I might get another better offer in the mean time but I'll be bound to this one since I accepted it...
Any advice would be helpful. Thanks.

Read the contract, it should entail the period of notice you have to give to quit from this job.

But don't be a dick and tell them you don't want the job days out from when you're suppose to start.

I had exactly this happen to me last year. I accepted a full time job in a place I was contracting for, to start in 3 months, contract had a 1 month notice period. About 2 months before, I got an offer that I couldn't refuse, so told the previous company within the one month that unfortunately I would not be taking up the role. All was fine. Just make sure you let them know within the notice period (probably 1 month or 1cweek before).

I know people that have done that but never done it myself. In those instances the employer they let down was pissed but they didn’t do anything.

Having You new job start in 6 months is a hell of a notice period.

I've done it multiple times, you might ruin your relationship with that particular company so do it as nicely as possible but nothing bad is going to happen legally. Worst case is an angry phone call or email.

If you have signed a contract then it's a potentially different ballgame and you shouldn't take advice from strangers on the Internet about it.