Thanks for your reply.
I think the problem is that in the UK the car rental companies use vague terms which do not always fit in with the USA classifications.
For instance NetFlights have the Ford Fusion as a 4 door Intermediate car but on the Thrifty.com website, the Ford Fusion is a Standard car and this category is not shown on Net Flights option. They have Compact, Intermediate and Full Size etc.
The voucher says Ford Fusion or similar but I don't think the Fusion is viewed as an Intermediate car in Florida. Do you see the confusion and difficulty when booking from the UK.