valid grounds for objection could include
the loss of jobs at the airfield (there are 3 flight schools at Enstone)
Loss of employment through change of use might well be a valid local planning issue, in which case it would aid credibility to quote the policy number in any objection. The risk is of course that the applicant will claim that the new use will provide the same number of jobs, or even more, thus eliminating this as a ground for objection.
the environmental and visual impact on the countryside
That's almost certainly worth trying, there's bound to be policies in this area.
increased traffic on the local roads
The local transport authority will have been consulted on the application. If they have problems with increased traffic then this one will be worth trying; otherwise, objecting to a planning application on traffic grounds when the local transport authority have said there isn't a problem very rarely has any chance of success. (I personally have seen it work exactly once, after several years of sitting on planning committees. And even that one occasion might simply have been because the applicant didn't appeal.)
the fact that there would appear to be no need or demand for more industrial units in that area
This is not usually a planning issue - if someone wants to start a business for which there's no demand that's their problem, nothing to do with the planning system. Unless, again, there's a specific local policy, in which case quoting its number in the objection would considerably aid credibility. If you do want to run with this one, "appear to be no need" isn't enough on its own, you need evidence; the planning authority might have done an employment land survey within living memory, that might have the figures you need, eg if it says that 10% of existing light industrial units have been unused for three years despite being actively marketed then that would be good evidence.
the detrimental effect that the loss of the airfield would have on the local economy (people using or visiting Enstone regularly use the local garage, shops, pubs and Heythrop Park Hotel)
Direct loss of employment through change of use might well be a valid local planning issue, as noted above, with the risk noted above. It's less clear to me that indirect loss of employment will work - it'll certainly need clear evidence, eg statistics from the hotel saying how many people have flown in who would not otherwise have stayed there, and how many jobs they would lose if this business ceased. However the applicant, if properly advised, would simply put in a counter bid saying that all the salesmen visiting the new industrial units would be staying in the hotel which would result in increased employment.
Also, whilst
the Council is not obliged to ensure that there are airfields available
is true in general and might well be true in this particular case it might not be universally true, as it depends on exactly what is written in the local planning policies, so it might work in other places.
There is no substitute for studying the local planning policies and coming up with planning reasons, with policy numbers, to support objections.
And, as always, the number of objections doesn't matter. If one person raises a valid objection to an application on planning grounds this has the same effect (on a correctly operated planning department and committee) as a thousand people making the same point.