Falling Penguin just to give you my example on Short Haul. I commute from Jersey and was at Heathrow for three years. Reserve was a pain but I was fortunate that my landlady in Horley from my days commuting in easyJet (Gatwick) had another spare room which she would rent to me for three weeks provided I gave her enough notice. Then, because of the early reports and the prohibitive cost of paying for a national express coach every time I was called, I rented a car for three weeks, which was usually pretty cheap. I used rentalcars.com which usually directed you to Interrent (the “low cost” offshoot of Europcar) which was pretty cheap.
I think the best price I got was £128 for 21 days of rental (cheaper and less risky than bringing my car over on the boat). The reason it’s so cheap is because they nickel and dime you with extra charges, like an extortionate rate for extra miles above the 90 miles a day they give you. 90 miles was fine for a Gatwick-Heathrow return trip so even if you were called every day (which doesn’t happen) you’ve still got a fair few miles to use the car for other things. In summary, it’s not easy and it’s a bit of a pain but that first year or so of doing reserve a lot will get expensive.
Long Haul I doubt you’d have to rent a car as long as you were staying in striking range of Gatwick or Heathrow as you could just National Express coach it from Gatwick or better if you’re staying around Heathrow just hop on the bus. No early reports and much fewer (I would imagine) last minute two hour call outs, which even then is still achievable from Gatters with the frequency of coaches up to Heathrow every day.