Just went for a nosey on your profile (nothing malicious), You already have at least 1 RIF & sidearm, you've made enough posts/comments to know how this thread would go. I'm going to assume you already have an UKARA or had one, just follow the cosplay rules at the event you want to attend.
Don't know about any list of retailers but a quick google produced this... https://www.just-cos.co.uk/ This is an airsoft forum & we occasionally have non airsoft questions like this that often turn sour in the replies. Not sure if you would be required to buy insurance for your hobby but it's certainly worth while spending a little time at an airsoft site to attain an UKARA registration (not a permit or a license) which will at least provide you with a 'defence' for having an imitation firearm, in my short time as an airsofter it's become apparent that some people (airsofter, cosplayers, reenactors) have fallen foul of UK law and provided the media with a very easy story (true or ficticious) which casts a very big shadow over our hobby.
What ever you decide to do please be aware of cosplay rules regarding firearms at events, enjoy yourself & be safe.
I'll repeat myself... it's certainly worth while spending a little time at an airsoft site to attain an UKARA registration, you'll probably find out you enjoy the experience, most of us dress up as a character when playing, some as milsim & some as ballerina's with pew pew's. You could appear at almost any site you'll have a mix of giggles & interest in equal measure in your direction but we all enjoy our day & you might become the first T-Rex badass at the site you chose to attend.