|Pyro as Bill said:
You're right. The price difference is probably closer to $50. SSD prices have come down more than I realised and 480GB and 1TB seem to the best price/size option.
Still, the amount of space needed in the first year isn't the same as in later years so I'd still take the £50 price drop with a 240GB/480GB and add my own USB SSD/HDD as and when it's needed.
Can Sony and MS's proprietary SSDs be replaced with larger sizes or can you only add to them?
On MS side probably no. There is a render of Series X motherboard with SSD shown in it and it seems like the form factor of this SSD is proprietary and not what you see in PCs. For PS5, we still don't know but I think it would use standard M2 SSD which would be user-replaceable just like HDD on PS4 was.