From the BA website:
" You can bring an unlimited number of wheelchairs or mobility aids in addition to your free baggage allowance.
At most airports, you’ll be able to stay in your own wheelchair until you reach the departure gate.
We can store one collapsible manual wheelchair in the cabin, where space is available on-board. If you’d like to do this, just let us know when you arrive at the airport.
To make sure we are able to take you to your seat safely and securely please arrive at the departure gate in good time.
It is important that we have as much information as possible about your wheelchair at least 48 hours before you travel, to enable us to safely and securely load your wheelchair onto the aircraft."
However it's worth remembering that BA is only one of the four airlines flying that route. The others are easyJet, Monarch and Thomson.
If you're going to hire a car anyway, it would seem sensible to collect it from the airport. There's a list of the car rental companies, which are represented at the airport, here:
http://www.aena-aeropuertos.es/csee/Satellite/Aeropuerto-Malaga/en/Page/1237553518661//Rent-a-car-desks.html
If you need assistance at Malaga Airport, see here:
http://www.aena-aeropuertos.es/csee/Satellite/Aeropuerto-Malaga/en/Page/1213253361509//Request-for-assistance.html
See here for information about assistance at Gatwick:
http://www.gatwickairport.com/prm/
Regarding hotels, this list might be a good place to start:
http://www.laterooms.com/en/k16855190_malaga-ciudad-hotels.aspx?q=2_disabled-access-hotels
Chris