How to find a mattress

Hello, I am new into the UK and I'm tight on the budget. Can't afford a really expensive mattress but I'm looking for some good options. I order mattress recently from Amazon 100£ But after a month, it's gone so deep. Like when I sit on it it's just completely goes flat. So can you recommend me where should I look for some mattresses which last at least 2 or 3 years and how to check before buying like when this mattress even came? It was so hard from the top like nice. Even if I go to the shop by myself and check it look nice. But what after 1 or 2 months it goes deep how to check it​