Moderator, Iberia Airlines, Airport Lounges, and Ambassador, British Airways Executive Club
Join Date: Feb 2010
Programs: BA Lifetime Gold; Flying Blue Life Platinum; LH Sen.; Hilton Diamond; Kemal Kebabs Prized Customer
Posts: 64,091
I'm afraid with 2 separate tickets you're only allowed that ticket's allowance, you can't read it over. One ticket would have allowed it all the way through. And you'll have to recheck the bags in LHR too.
If your fare category allows it, have a look at upgrading using Avios one of the Belfast sectors to Club Europe, that would probably nearly pay for itself. If your fare basis is too low (Q, O, G) then you could pay for a POUG (if offered online) or an AUP at the airport for approx £65.