W7 do a HD one for £3, it's really good and my go to cheap foundation for day to day. It's decent coverage, better staying power than some of my more expensive ones and for £3 it's a bargain.
If you are near a Wilkinsons, they have a skincare range exclusive to them and have a lovely primer for £3. (Pictured) Its gorgeous, reminds me of the benefit one.
My fave foundation though is the Max Factor Age elixir one. Its a foundation and serum in one, lasts all day and night, (fell asleep without removing once and it was perfect the next day). It's my Holy Grail, the only foundation I have that perfectly matches my skin colour, gives full coverage but looks like my natural skin, and I do t need primer or concealer etc when wearing it.
It's £13 in boots ATM, but last week they had an offer which may still be active where if you spend £15 on max factor products you get a free set with full sized masterpiece mascara, a full sized blush a full sized lipfinity lipgloss.
The no7 foundations are good too, they also have offer where if you buy two no7 products you get a free Spring make up set containing no7 blush, lip crayon, mascara and skin illuminator. :)