Going to take some convincing that it'll be cheaper to hire:
Life Jacket Hire - Tool Hire, Equipment Hire, Lifting Hire, Plumbing & Pipe Hire - Brandon Tool Hire
I realise they are aiming for a different market. I'm sure one of the London slips said they hire recently.
If you are confident that you don't need need any fancy stuff (lights, spray hoods, hammar) you'd probably get a basic auto for about £40-45 each which is the equivalent of £8 a day. Plus you need to pick them up and return them and unless its local you end up with extra days of hire there.
If its possibly cheaper I'd be surprised and if it is I'd guess it wouldn't be the following year if you needed a repeat...
Maybe better to speak with a rib rides company and see if they have old stock they are clearing that would be cheaper still...