Maximum upgrade is to a Full-size car with a minimum reserved Compact vehicle. For Compact 4-Door rentals, maximum upgrade will be to a Full-Size 4-Door car. Guaranteed upgrade is to the next car class only. This excludes Special Equipment such as Minivans, SUVs and Hertz Collections vehicles. Upgrades do not apply to rentals longer than 30 days.
So you have no entitlement to anything bigger than an F/X, whether you book a D/E or an F/X (using UK categories).
Of course, if they have two F rentals (or someone else has an entitlement to an upgrade into an F) and only one car, they will upgrade you first if booked on F, but if there's plenty of stock, you'll drive an F.
I tend to book C all the time because the price difference over A/B is small and I'm happy to take a Focus or similar if I have to. In general though, my reservation is treated as though it were for a D class car, meaning that I'm likely to get upgraded further no matter what as there are currently few Ds at LHR and I'm at the head of the upgrade queue. However, rather than end up in an F/G/H I am quite likely to end up in a Qashqai, Galaxy, or some random thing that was hugely overpriced and therefore was only going to sit around.
I used to book Ds occasionally, but only ever got one premium upgrade and that was from Sunbury (where they get cars from LHR but you don't get the status upgrade anyway, plus I think I was 5* then).
Personally I'd book C or even D.