Amazon UK - Save on HDDs, SDDs, and external drives
Scan - SATA drives often as low as £17
Box - Save on SSDs, including Western Digital's SN750 NVMe
Ebuyer - SSDs starting from around £30 for 512GB
Overclockers - Lots of SSDs to choose from
Currys - Plenty of options from SATA to NVMe
Laptops Direct - Surprisingly not just laptops, SSDs too!
CCL - SSDs for bargain prices
Box - Internal and external drives