Most decent UK EVSEs do include DC protection, and also PEN fault protection.
(Recent legislation also requires any CP sold in the UK now to have a specified set of smart functionality).
This makes sense as the EVSE needs a contactor/relay to switch power to the car anyway, so only the detection is needed.
The problem with doing it externally is that type B RCDs are quite expensive and not widely stocked. I don't think anyone makes a type B DP RCBO, so if installing in a typical UK consumer unit it's going to take up 3 slots.
Another issue is that there is that there is a common perception that UK wiring regs imply that you can only install breakers from the same manufacturer as the CU, though this makes no objective sense provideds the busbar height is correct.
There is also some scope for interpretation around the need for a DP RCD, as the EVSE will always have a DP contactor which will disconnect if power is lost, so the wiring regs requirement for DP RCD protection could be interpreted differently depending on exactly where you draw the line between the RCD and the car connector.
There are cheaper Type B RCD options, like this, which I have tested and taken apart, & seems to be reasonable :
https://www.aliexpress.com/item/1005003085547942.html?spm=a2g0o.order_list.0.0.5a0b1802lEFL1m