As none of the measurements are less than the width of the door opening, it doesn't seem it will fit.
A Bank of England Five Pound note (Series D -blue)(serial RE36 - Chief Cashier G M GIll) uncirculated and in absolute mint condition could fetch anything up to £25 GBP. If it has been circulated but still in good condition, it might fetch anything up to £10 GBP. Alternatively, they are worth £5 GBP if returned to the Bank of England. A reputable coin dealer will be able to give a more accurate valuation.
It is screewed into the side of the engine, just in front of the starter. when you remove it coolant will come out of the engine. Just be fast about changing it and make sure engine is cold.