Over the counter MCAS treatment

I'm looking for OTC MCAS treatments to try to see if they have any effect. My doctor has left me in charge but without the prescribing power.

I'm under UK jurisdiction.

H1 blockers are widely available. What about H2? Can't seem to even get pepcid without a prescription.

Mast cell stabilisers - Quercetin gave me a bad reaction before (actually, it also had olive leaf extract), anybody know a possible reason why?

Any other stabilisers which are supplements or herbs etc that work?

Cheers!