Hi Frank15 and welcome to the Mattress Underground 
Sorry to hear your old foam mattress is no longer comfortable for you. As you likely know, any mattress you choose will be entirely unique to you, based on your Stats (height, BMI, sleeping position(s) and any underlying health conditions) and your PPP (Posture & alignment, Pressure relief, and Personal preferences). Thanks for providing these! As a side sleeper sleeper in the normal range of BMI, you have a lot of options open to you.
First, are you just looking for another foam bed? Or something more durable? Foam has its’ own pros and cons…you do say you sometimes sleep ‘hot’, which is not uncommon for memory foam, as by design it conforms to the sleepers’ body and thus keeps body heat close to the surface of the skin. A more breathable topper or cover can sometimes alleviate this ‘hot sleeping’…For foams we suggest any foam in a potential mattress has no more than an inch or so of lower grade foams (for memory foam no lower than 4 lbs/cuft for normal range weights, and 5lbs/cuft for higher range weights) and if polyfoam is used, with at least 1.8 lbs/cuft density, just to ensure you have support. Are you considering other materials like latex? You can compare the Mattress Specifications You Need To Know to the Mattress Durability Guidelines to see how durable any materials will be over time - for example, latex is one of the most durable mattress materials you can find, though not everyone enjoys the feel of latex. An inexpensive foam mattress can break down in a few years. For your stated budget, you should be able to find a comfortable and durable sleep solution.
Unfortunately, due to all the global economic and supply chain issues over the last few years, it’s not possible for us to provide any geographic listings for UK mattress retailers and manufacturers. You can do a forum search but most of the info is unfortunately a few years old.
If possible you should try some different mattress types - foam, latex, hybrid - at differing thicknesses and firmnesses to get an idea of what kinds of mattress you are looking for at any local shops, and then look at the options for your ‘short list’ of mattresses you are considering…then you can look at each to see how transparent they are with their materials, composition and construction, and how they may or may not work with your Stats and PPP.
You may also want to read the Mattress Shopping Guidelines to see how to locate a good mattress outlet…and if you find any specific mattresses you are looking at, and provide your Stats, we may be able to help assess how it might work for you and your sleep profile. Hopefully other forum members from the UK can weigh in on their experiences as well.
Cheers,
~ Basilio