To the above figure one must add the obligatory cost of owning/renting an apartment (+ health insurance). Even a tinydoghousestudio costs close to £1,000/month, so the real minimum cost of "a residency with no physical presence" is more like £33-36K/year.
You actually need way more than £1,000 rent a month more like £1,600 at least. The property you rent in Gibraltar or buy must specifically have CAT 2 qualified status. You cannot rent or buy any random property and apply for CAT 2 status that is forbidden. Health care is cheap in general. So you need a minimum of £40,000 all in plus food, utilities etc. You must also prove your way north of £2m will produce a stable income to support that 40k cost and maintain your self sufficiency otherwise you won't get in.