Should I expect extra fees going through rentalcars.com

I can't speak specifically for the UK and what kind of fees they may add on, but we rented with Europcar (via AutoEurope) in France and when we dropped off the car there were extra fees...we paid extra for picking up at the TGV rail station (34 euro - which I knew about beforehand by checking out the Europcar site). There were also fees for dropping at an airport (we dropped elsewhere to avoid the fee).

There were also daily fees of a few euros for different things and dependent on how long you had the car. Unfortunately, the final drop bill was in French, but there were a few daily fees of a few euro a day - one was an ecological fee of 4.50 euro per day, a flat 15 euro fee for something (indemn. carburant - whatever that is). The UK could have different regulations, but don't be surprised to see extra fees at the end.

For 9 days, we paid an extra 115 euro (+ 20% tax) for fees.

Check your voucher from rentalcars, assuming you got one. As did Nicole, we've frequently rented Europcar through AutoEurope in France, and the voucher clearly spells out the extra charges, including the various daily charges,.