I think it's a rubbish excuse - they use the same 73x fleet everywhere at LGW (for mainline) so they'll be no 'smaller' than when they fly any other service on the network! But I recall when the SZG route launched we couldn't get seats for ages for the 'do' (which was on/around launch time depending on when people flew) and then suddenly pre-assignment just worked. I think it may just be teething problems when new routes are launched? OK, you're getting fairly close to departure now but I'd keep checking - you never know...

...and if not, I suspect there's nothing anyone can do so everyone is in the same boat - no other Silver/Gold will have pre-assigned the best seats by now either, so hit OLCI at T-25 if you can.