I am also in the UK. You can use supermarket or drugstore brands, just read the ingredients, how high or low the protein is in the list, what the other ingredients are. Just because a product says it's moisturising or repairing or whatever doesn't actually mean it's doing much. Products don't all need to be for curly hair, but some ingredients may aid in curling and some may straighten. Personally I combine higher end brands with cheap products to get the ingredients I want.

Particularly beneficial ingredients for porous or colour damaged hair include coconut oil, hydrolysed protein, ceramides and panthenol. I am currently liking Schwarzkopf Gliss Ultimate Volume conditioner, you can find it in 1 shops, BodyCare and Home Bargains. It's not a super slippy conditioner, don't expect to use it for detangling.
"Aqua, Cetearyl Alcohol, Hydrolyzed Collagen, Panthenol, Cocodimonium Hydroxypropyl Hydrolyzed Keratin, Hydrolyzed Keratin, Isopropyl Myristate, behonoyl PG-Trimonium Chloride, Citric Acid, VP/VA Coplymer, Phenoxyethanol, Behentrimonium Chloride, Sodium Methylparaben, Hexylene Glycol, Parfum, Polyquaternium-37, Dicaprylyl Carbonate, Hexyl Cinnamal, Amyl Cinnamal, Limonene, Linalool, Benzyl Salicylate."

You can still find the old silicone free formula Garnier Fructis Sleek & Shine conditioner in 1 shops, not as much protein as the Gliss but has more 'slip' due to the benhentrimonium
"Water, cetearyl alcohol, palm oil, behentrimonium chloride, niacinamide, sugar cane extract, hydrolysed corn protein, hydrolysed soy protein, hydrolysed wheat protein, stearamidopropyl dimethylamine, chlorhexidine dihydrochloride, xylose, limonene, camellia sinesis leaf extract, linalool, benzyl salicylate, benzyl alcohol, isopropyl alcohol, apple fruit extract, pyridoxine HCl, citric acid, butylphenyl methylpropional, lemon peel extract, hexylcinnamal, glycerin, fragrance FIL C35637/1"

I also rate the Komaza Care line: the protein strengthener (hardcore) and the Matani repair treatment spray (less intense, also contains ceramides and panthenol) but that is an online purchase. Your final option is to try the DIY gelatin treatment, you can get gelatin in any supermarket.
2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect