Booked Thomson flying Jet2

[TheD]

I booked to go to Cyprus with Thomsons. Wife did it because I prefer to do it all separate. Anyhooos.....Turns out although it was all booked with Thomson we have found out we are flying with Jet2. I actually like Jet2 so no problem there. The problem is we can't book our seats until the airport. Also it seems you only get 15kg hold baggage while if I booked Jet2 I would get 22Kg. If I don't pay Thomson the extra money for more weight will the Jet2 check in notice.
Phoned them and got a nice Indian fella who knew nothing....nothing at all.