From experience...........Get in early now and look for a local one.
We tried booking an 8 seater for sons school prom in Stafford about this time a couple of years ago for the July and all the Stafford/Stoke/Cannock /Wolverhampton etc ones were booked up.
Then they found a national one off the internet (Something like Wessex/Sussex/Essex Limo Hire) who said they did this area,and as well our lads, they amazingly had Limos available for a few other groups of mates as well. Sounding too good to be true I refused to book it with my debit card, but one of the lads fathers booked it and paid with his Credit card . A couple of days before the prom one of the lads went to show someone the Limo they were having. The website had disappeared, phone number not available and turned out it was a national scam which the fraud squad were investigating!
At the last minute we managed to get a firm from Stoke on Trent (sorry, can't remember the name) who offered, for a reduced price, to pick the lads up and take them straight to the Prom after they they had dropped another party off at the same school. Ended up being the last ones to arrive at the Prom (not helped by a delay through the motorway being closed that evening) and missed out on a ride around beforehand, but better than nothing ...................................... some others who had booked with the scam people weren't so lucky and had to walk it in.
Turns out the 'After Prom Party' organised at a mates house was much better than the actual prom and son's suit from Asda looked just as good as the ones they had spent a fortune on. ( And he tried it on again last night when I was having a sort out and still looks good on him, so saves him buying one for his Leavers Ball this year!)