I think getting off at Watford is a fairly sensible option. There are always taxis milling around there, it would be very unusual to not find one.

You could even pre-book a mini cab, all the local firms will have a published price for a drop off at Heathrow, my guestimate would be around £40.

Another option from Watford is the 724 bus, but I doubt you're in the market for that.