Fuel weighs less than 1 kilo per litre
(Gasoline weighs about 6.1 pounds per gallon (lb./gal.) (728 g/L).)
so a full tank at 55 litres weighs 40.04kg
this is only 3.1% of the total weight of the car
in a car weighing 1285 kg is the extra going to make that much difference?
This may be of interest if you want to save fuel
http://www.save-petrol.co.uk
FYI your £25 pounds worth will be about 22L @ £1.10/L and will weigh about 16KG, assuming your tank is dry when you buy you are saving 33L or 24kg approx.