During the same period that Spridgets were young, the UK local authorities had a hair-trigger approach to potential freezing in the winter, so salt - lots of salt - was the order of the day. ALL cars of this era were made without any kind of corrosion protection, so the lower parts rusted out very quickly - and there wasn't much you could do about it, the undersealing that was available was only somewhat effective (speaking from personal experience).
I would guess that a rust-free Spridget (especially a Bugeye) would be rare in the UK unless it had been VERY rarely used in wet weather, kept in a heated garage, and lovingly coddled for those 40+ years.
I am fortunate that mine spent its youth in California, and its middle age in a garage in Raleigh NC, rarely venturing out. Needless to say, this latter was not the way most UK Spridgets were treated.