I am afraid you have misdirected yourself. The definition of whether a place is indoors for the purpose of the regulations at hand, in section 2 (6) (f) of SI 2020/1374 (as amended) makes reference to whether it would meet the definition of enclosed in section 2 of the Health Act 2006. Section 2 of the Health Act 2006 makes reference only to premises; vehicles are defined and made smoke-free pursuant to section 5. It is clear, therefore, that for the purpose of SI 2020/1374, a vehicle is not indoors, regardless of whether or not it possesses a roof.
I would absolutely like for this to be the case, but I don't share your optimism! Unfortunately the legislation is very vague on this point, using the phrase "place" (in SI 2020/1374) almost interchangeably with "premises" (in 2006 c. 28).
Sec. 2 of the latter subordinates the definition of "enclosed" and "substantially enclosed" to secondary legislation. It makes no reference to that only being for defining "enclosed
premises" etc. - and neither does SI 2020/1374 reference Sec. 5, so I don't think that Sec. 5 has any relevance to SI 2020/1374.
SI 2006/3368 takes up the opportunity given by Sec. 2, but rather unhelpfully only defines what "enclosed
premises" etc. are. Therefore it is unclear whether the term "enclosed
place" etc. is actually defined for the purposes of SI 2020/1374, rather than taking its ordinary meaning.
It's a total mess. Realistically speaking, most police officers are going to treat people that are together in a vehicle as holding an indoor gathering.
If those people were issued with a FPN, it's questionable whether it would be a productive use of money, time and effort to fight the matter in the Magistrates' (who would no doubt convict), then to appeal, etc.