Eighteen years of your safe list shrinking, and now you're eyeing the eggs too — I really feel this one. I'm on a very restricted, mostly-liquid diet myself (different underlying cause, but the same "what on earth is left" spiral), so let me answer your actual question first: how to get proper help beyond the GP.
• The person you want is a registered Dietitian, not a "nutritionist" — "dietitian" is a legally protected title and they're HCPC-registered, whereas anyone can call themselves a nutritionist. For private or remote appointments, the British Dietetic Association keeps a freelance directory (freelancedietitians.org) you can filter by specialism (IBS, gut, allergy); a lot of them consult over video now, which helps if getting out is hard.
• To get it on the NHS, the words that unlock a referral aren't "IBS" — they're that you're unintentionally restricting your diet and at risk of nutritional inadequacy / unplanned weight change. Framed that way it's a gastroenterology-plus-dietetics referral, not "have you tried peppermint tea". And since this all started with the interstitial cystitis, it's worth asking whether your urology/IC team can refer too — they'll understand the bladder side that a general GP might not.
I'd be cautious about spending money on: private "food intolerance" blood tests (the IgG ones) and hair/Vega testing. They aren't validated, the BDA advises against them, and — this is the real harm — they nearly always hand you a longer avoid-list, which is the exact opposite of what you need right now.
Two practical things to bare in mind
• Don't drop the eggs on suspicion alone. They're one of your few proteins, and when you're this restricted, cutting another whole food is a big deal. Keep a simple diary first — food, time, symptom, time — for a couple of weeks. Days-delayed reactions (which yours sound like) are notoriously easy to pin on the wrong thing.
• A good dietitian's job here isn't to police what's left — it's to help you add foods back. After years of restriction the list often shrinks partly from a genuinely twitchy gut and partly from very understandable fear, and careful structured reintroduction (tiny amounts, one thing at a time) really does widen it for a lot of people. That reframe — widening, not just surviving — was what shifted things for me.
Two other things: with no dairy for years, do ask for your calcium, vitamin D, B12, iron and ferritin to be checked — long restriction can cause deficiencies , and that alone can make everything feel worse. And on the IC side specifically, the standard bladder approach is elimination then phased reintroduction (a surprising number of "trigger" foods come back in small amounts); some people find calcium-glycerophosphate (sold as Prelief) takes the edge off acidic foods — worth an experiment, not something I'd build a budget around.