If a break clause is expressed to be personal to a particular T, then that break clause will only be exerciseable when the lease is vested in that T (ie it will not be exerciseable after T has assigned and is therefore the former T). Indeed, for a former T to exercise a break clause would require ‘an unambiguously clear’ wording (and it seems that there is no reported case in which that was so).

If T does have a personal break clause and wants to keep its options open for the future, then the best advice is for T to consider under-letting rather than assigning. T would then be able to exercise the break, and that would also end the under-lease (but do make sure the under-lease is contracted out of LTA 1954, since otherwise the under-T would be able to apply directly to L for a renewal lease). See Linpac v Aviva [2010] EWCA Civ 395.