Where can I buy cheap plain tops/trousers?

Vinted is good

EBay. There is so much choice and shouldn't be too expensive 

Another vote here for good old Primark. Their clothes ranges intended for exercise are generally plain colours and super comfy- I'm wearing a pair of their leggings right now. Can't guarantee they'll have blue but they do often have darker blue things available.

I find Primani  clothes very soft feeling and cheap. I buy most of my clothing from them. 

On the mainland, it's often referred to as 'Primani'. 

According to Urban Dictionary, it is an oxymoronic amalgamation used by shoppers of the Primark clothing chain, in order to add an illusion of quality and high class.

Pronounced Pre-Mark; here, in Northern Ireland.

George at Asda.

If you haven't already done so, it might be worth seeing what the supermarkets with clothing departments have to offer. Another suggestion is Primark.

Also all the branded shops, have and end of summer sale atm, at about 50%..