so i could upgrade my pc i have now by doing a case swap and getting a new motherboard then i have the cpu ram and ssd and i already have a 1tb hdd for extra storage so i just need to get a motherboard and case and gpu and a new psu with £350 but i dont have to get a new cpu
If you want the best bang for the buck pound you have to think creatively, like you're on Scrapyard Wars.
Here's a 9020 tower with an i5 and 8 gigs of RAM for £60 plus postage:
You could get that, a power supply, and a nice video card. Then swap your CPU, RAM, and storage into it, and put the tower's parts in the SFF machine. Then you can resell the SFF machine for around what you paid for the tower (after doing a clean Windows install on it). Bam! There's your case swap for almost free (plus some elbow grease).
EDIT: Budget £13 for a power supply adapter cable.