You could compare car insurance quotes using the Free Price Compare car insurance calculator to try to find a more favourable deal. You should be aware that the owner of the car is unlikely to want you to opt for a cheaper policy that requires a black box to be fitted to the vehicle, particularly if you are not likely to be driving it regularly. This is because any poor driving behaviour when you are behind the wheel could impact the price of their premiums.
Alternatively, you could investigate the possibility of renting a hire car from a dedicated hire car company. Many car hire companies require renters to be aged over 25, however, some will consider younger drivers who have held a full UK driving licence for a year or more [1] though they will apply a Young Driver's Surcharge to the hire fee which will increase the price.
If all else fails, you may have to buy your own vehicle, or if you already own one that is off the road, prioritise making it roadworthy. Nobody wants to have to rely on public transport and the benefits of owning a vehicle that is covered by a fully comprehensive car insurance policy are huge.