I inherited a Royal Enfield pushbike from my Dad YEEEEEEEARS ago. Had rod brakes and everything.
But yes, BSA did indeed start off making guns. Lots of guns. https://www.bsacompany.co.uk/history/
I know, was tongue in cheek here too!
I like how they started off doing pushbikes and sewing machines and stuff and then just went "fuck it, let's make guns again!"
According to MCM's own website airsoft guns are banned from all their events so...
https://www.mcmcomiccon.com/london/en-us/costume-weapons-props-rules.html
Maybe the local security haven't been kept in the loop.
I currently have four guns I've never skirmished (yet) and loads more bits and knick knacks that I've bought because they seemed like a good idea, tried once or twice and then consigned to the drawer of doom (final resting place of three different flip to side magnifiers for example).
Where are you finding decent LiFe stick packs? They were all the rage for airsoft a few years back but most places only seem to have one or two choices these days (or are you repurposing RC packs?)
My go-to? A Platatac Peacekeeper MK1.
Can carry 6 7.62 mags or 9 5.56 ones plus pyro, water, the whole 9 yards. Not cheap though.
Also - you haven't told us what you use, what sort of games you play etc.