There is no rule against DEET as far as I know, either in the cabin or in checked bags. A lot of crew carry 50% DEET in pump bottles. There are rules about aerosols though:
Aerosols containing non-flammable, non-toxic gas with no subsidiary risk (such as nitrogen or carbon dioxide), for sporting or home use
The total net quantity of each single article must not exceed 0.5 kg or 0.5 L.
The total net quantity of medicines, toiletry articles and aerosols for sporting or home use must not exceed 2 kg or 2 L (e.g. four aerosol cans of 500 mL each) each.
Release valves on aerosols must be protected by a cap or other suitable means to prevent inadvertent release of the contents.
Carry-on Baggage NOChecked (hold) BaggageYES On One's Person NOAirline approval required NO
https://www.caa.co.uk/Passengers/Bef...ed-in-baggage/