They don't come up so often on
eBay now but you could, and still can from time to time, get something like a Louis Garneau Prologue helmet. These can go for around £25-30. Worth setting up an automated search for anyway.
I had one and I liked it very much, it was destroyed in an incident with a 4x4. It wasn't on my head at the time, fortunately.
In some ways I prefered it to the more expensive MET aero hat I have now since it came with a visor. The tail of the Prologue doesn't sit flat on your back- it sort of sticks up a bit- but having the visor is probably more important aerodynamically.