Because the RSGB is run by tedious atavistic old farts, who want the hobby to be an Old Men's Club forever.

Meanwhile Ofcom's unofficial position is that they literally cannot be paid to investigate things that happen on the amateur bands, and don't much care what you do anywhere else as long as you're not making a nuisance of yourself.

Formally - no, i think the official stance last i saw was that you can submit a request for a “notice of variation” on your licence but i don’t know of anyone who’s done that and from my knowledge of other NoVs that have been granted i expect it could be hobbled in some way. E.g. in the uk a full licence holder may transmit at up to 400w on most bands but you can submit for a NoV to be able to transmit at 1kw like the US - except the NoV can carry constraints like you must not transmit your call sign. That’s not a typo, you must NOT transmit your callsign when using 1kw. It’ll depend on the reason for the NoV being issued though.

Informally i’ve had nothing but condescending dismissal on airborne operation because i mentioned some experiments involving a drone (my other hobby).