I had found a decent fare on ba.com for Business class flights to Florida during the Easter hols:
Outbound: GLA-LGW-TPA in Club World
Inbound: TPA-MIA-LHR-GLA in Business on AA

£6,388 for family of 4.

As I will be hiring a car, I tried to do a BA Holiday ‘Flight + Car’ package in order to just pay the deposit (and save triggering my Amex 241) however I was continually getting the error: ‘Unfortunately our systems are not responding, so we are unable to process your request at the moment. We apologise for this inconvenience and suggest you try again later.'

I called BA Holidays and attempted to make booking over the phone, however the agent was unable to see the same flight routing as I was online (and hence unable to offer the same fare).

After a long conversation and trying different options, I discovered that when looking to make a flight + car booking, depending on whether it is chosen as 'Flight + Car' option at outset or straightforward flight, then add car at the end, different flight options are shown and the fare I was quoted online for Flight then add car at the end is not available if doing a 'Flight + Car' option (which goes through BA Holidays, rather then BA direct... excuse me if I have this wrong but this is how I understand it).

So the fare I can see (as above)+ car for £506 is not available to book online (as I continually get the error message about system not responding when i get to the payment page). BA Holidays are unable to book this as according to the agent the flight routing does not come up as an option. When I search for a BA Holidays 'Flight + Car' option, the only fare they have is £9,477 which seems to be pricing up only fares for GLA-LGW-TPA, TPA-LGW-GLA with BA (ie not giving the lower fare option to route via MIA).

So I could book the flights separately, then the car but I would have to pay the full amount just now, rather than the 10% deposit with BA Holidays (and therefore avoid triggering my AMEX 241 ).

The BAEC Agent was very helpful, i just don't understand why BA.com will sell me the flights but BA Holidays can’t offer the same flights + car booking as a package .

Anyone in the know enlighten me more on this or offer any suggestions?