exxb downgrading as you refer means 'class of travel booked'
ie W downgrade to Y
C downgrade to W
does not apply to optional extras - the OP was not booked
afaik in the Thomson premium seating which would likley carry a C or W
all he did was pay for extra leg room in Y
standard seating TOM l/haul is 33'' if he got 34'' i would say they is taking the p***
and he deserves his money back in full- just keep badgering Thomson and tell your credit card company and ask for a full refund back to your card of those charges - by reason of not fit for purpose - found out whether row 12 is designated premium seating and what seat pitch it has
if nothing else works advise Thomson with small claims court lines