If they advertise a TC commencing in 2027, they likely want you to start the SQE in 2026, so as to study together with your trainee cohort and to be able to start the job immediately afterwards. For the second questions, they may be able to consider switching you to the 2026 cohort; but from my experience they will likely prefer not to and I would not raise my hopes too high in that regard. This is a situation firms often find themselves in when recruiting from different year groups and they know most people do not want any breaks between university, SQE, and the start of the actual TC. As such, to avoid "opening up the floodgates" of earlier start requests, they might have quite a strict policy in this regard.
That said, I know of multiple cases where people managed to successfully negotiate this. Factors that can help for that purpose are holding another TC offer (as this will give you significant bargaining power) or there being dropouts from the 2026 cohort. However, there is no harm in asking (so long as you do it in an appropriate manner) even in absence of that. Worse case scenario, if there is no possibility to start earlier, you will have a good chance of perhaps securing a paralegal opportunity for the free year.