Is it illegal to sign a contract with an apprenticeship then go back on it?

Hi I recently received 2 offers from apprenticeships and one of them is asking me to sign a contract, within 5 days, on the formal letter of employment if I would like to accept. Issue is the second offer I haven't seen the contract yet (will be sent within 2 weeks) and neither of the 2 are my top choice. I am currently at the final interview of 2 others I feel will be better than the one I have currently and I also have done the final interview of another apprenticeship which is my top choice but I am awaiting their decision (it has been 3 weeks). I want to keep my options open and I don't want to reject this as of right now in case I don't get into any others / the offers aren't as good. Am I, as a 17 year old, allowed to sign the contract then go back on it should I receive an offer? And what are the implications of this? Thanks.

Read the contract you are being asked to sign. What provision does it make for you terminating the contract? Does it even allow you to terminate the contract early? If there is no provision with the contract which would allow it to be terminated, then you would be in breach of contract if you attempted to simply walk away when a better opportunity came along.
Generally speaking, a minor (i.e. someone below the age of 18) can void a contract they enter into at any time - simply because they are not deemed old enough to understand it. (See the Minors’ Contracts Act 1987.)
However, there is a understandable specific exception for education and apprenticeships. So you cannot use the "but I'm only 17 argument" to wriggle out of it. Such a contract is binding.
Your best option, if there is no specific provision with the contract which would allow you to terminate it, would be to try to stall them until such time as you know you want to sign it. You shouldn't be signing a contract which you know you're likely to want to walk away from.
All that being said, the consequences of walking away after having signed it are not likely to be significant. The other party could take you to court and seek reparations, but in reality they are unlikely to do so.
(Caveat: Not a lawyer.)

Hi I’m in the similar position as OP I have signed the contract as I was waiting for results day (tomorrow) and they wouldn’t let me wait until then so I could make my decision on my offers, the offer says it is conditional I pass the background checks which I have not yet completed so if I do decide to turn back on my offer tomorrow there will be no legal implications? As I am not yet legally binded to that contract yet as it is conditional and I haven’t met all the conditions yet? Please help me out lol but I feel like all the rejections I’ve been getting is fair for me to just reject the company but it’s life I guess

If the offer is conditional on you providing information to allow them to do background checks (e.g. previous addresses), and you don't provide that, then they can't do the background check. But that doesn't prevent them from skipping the background check and holding you to the signed contract anyway.
I guess you should just wait and see what tomorrow holds.

Yeah true hopefully they understand that I do have other offers and that this is a game at the end of the day like I told them that results day is when I’d let them know and they would do the same if there was a better applicant than me they wouldn’t think twice to just disregard me so it goes both ways. Have you seen when the company have like taken it serious and legal implications been done before? Any advice on how I should say it? I’ll say something along the lines “I regret to inform you that I have changed my mind on the opportunity and would not like to go forward with the is apprenticeship. Sorry for the inconvenience.” ?