As others have stated, these cars came from an era when rustproofing as not a high priority. My book — Jensen Interceptor, the Complete Story, by John Tipler, says:
The basic chassis tube assembly was built in a day, brought into the factory on a jig, and AT LEAST 800 separate pieces of metal were welded onto it over the course of 4 weeks. Then they hung the doors and hatch lid, and finally 5 men went over it with lead to fill gaps and flaws - about 120 sticks of lead filler PER CAR. The car was dipped in an acid wash to clean it, then spent a week in the paint shop.
The wiring, power train, interior, etc were fitted over the next 12 weeks.
Sadly, over the course of several changes of ownership and bankruptcy proceeding, virtually all the tooling was lost or destroyed. So later cars were hand-built in the most laborious way imaginable. Each was subtly different and parts (or spares) had to be fitted, cut, trimmed or adjusted.
Restoring an Interceptor means a “time & materials” process - no sensible shop would quote a price in advance.
Front end body panels
Front end chassis structure coming together
Convertible in body shop
It’s a hand-built car constructed by British union workers in the Seventies - need I say more? There were 16 unions at the company; about 20 employees per union. Even the 6 secretaries in the company got their noses out of joint over having to clock in each day, and formed their own union in protest.
Can you imagine buying such a car?