I'm also a wary about speculating, but quietly have my money on (1) or (4).
The attack method du jour by Russia and now Iran is to hire local numpties to commit crimes/disruption. The numpties don't always know exactly who's hiring them or the full details of the job. They get told: set fire to this address; graffiti this slogan on that building.
Information now in the public domain:
– vans with a fake fuel company name;
– the police bailed the 5 guys caught on site;
– the apparent fact one of the guys called 999 an hour before the farmer;
– some other person, a British/Iranian dual national, has now been arrested in London "on suspicion of preparation of terrorist acts".
https://www.bbc.co.uk/news/articles/cmdx5xw36492o
So of course I don't know, but one could construct a story which fits this info whereby the British/Iranian national was involved in hiring a bunch of petty thieves to go do a fuel robbery at RAF Fairford, providing them with branded vans and a map location – but not the info that it was a military base.
The organiser's intention being that the break in to steal fuel would constitute an attack on the base.
It could be that the numpties got there, twigged what their target was, and got cold feet. Then one of them got so concerned he called 999, or he was undercover and always intending to raise the alarm.