Hey @Jessica Booker , I'm in the exact same position and am not from the UK. I therefore have already invested a lot in monetary terms towards my visa and bookings for the vac scheme.Ultimately, if there is nothing contractually stopping you then you can go ahead with the vacation scheme.
However, if you are certain you will accept Firm A's TC over anything Firm B would offer you though, I'd encourage you to withdraw from the vacation scheme unless earning the money is vital for you. There really wouldn't be any reason to do the vacation scheme if you are 100% sure you will accept Firm A's offer, unless you needed the vacation scheme's salary.
Backing out of the scheme would be a financially v bad decision for me. Do you think I should tell Firm A that I'll accept their offer and am committed to them. but need to do this vac scheme with B as I'm too deep into this. Would Firm A allow this at all? Or is it best to keep mum? (Just wondering if I could risk my TC offer by not disclosing. Though my contract has no restriction)
Thanks!