Contract cord carpet is very cheap and hard wearing, it comes cloth(felt) or latex backed these days which is much better than the foam of old which died within months. Usually had a choice of green or blue but more colours these days. We did a whole house in it as a "temporary" measure which ended up staying down for over 8 years.
Used to be very cheap at B&Q of all places (varied between ¬£1.75 and ¬£3 sq/m and we used to drag a OAP with us to get it 10% off on Wednesdays) but I don't think they do carpet any longer?
On some rooms I put grey foam sheet underlay down underneath, it is intended for laminate flooring but is cheap and much longer lasting than foam carpet underlay and takes the hardness out the floor.
Both the other carpet warehouses we used to use have closed so I'm afraid I'm not very helpful on that front.
Plenty on ebay at less than ¬£3 sq/m eg HERE
For cheap carpet stick to proper small loop cord, the other twist and loop piles don't last as long.
If you are planning on spending more then its all a different ballgame.