The 16 venues for the WC have a total capacity of 1.02m. Qatar had 8 stadiums, capacity 385k .
UK top 16 stadiums currently has a total capacity of 1. 014, without including Dublin or increases in 3 of those stadiums. So, yes, UK has the capacity to host a WC.
Nothing says “British people imagine Britain to be the world” quite like the suggestion that smaller stadiums will be ok for a world cup in Britain because Britain is smaller.