Hi
Corgi/gas safe registered heating engineer here.
Firstly, do not use British Gas if price is your prime concern. Yes they do a top quality job, they do it quick, you have comeback with them and they offer finance but they are going to be at least twice as expensive as a smaller independent firm. There are plenty of ex-british gas engineers out there who now work for themselves and who will do a good a job as B.G.
Secondly, without surveying the site it is impossible for me to say if your new boiler can go in your old location. Your old boiler must have had a flue leading to the outside so there may be a way the new boiler can be installed in the same place. You have to be aware though that the new condensing boilers need to be near to a drain point for the condensate as well as needing an outside wall for the pressure relief pipework.
the usual place to install a combi is where your hot water cylinder is as that will no longer be needed and all the pipework except for the gas will be present, again though an outside wall is needed.
I'm not a big fan of boilers in lofts as personally if i have a machine in the house with gas, electric, water and live flames i like to be able to keep an eye on it. Also if the boiler is in the loft which is the highest point of the heating system if the system was to lose water then the boiler could run dry and ruin the heat exchanger or at least keep cutting out due to the low water pressure protection device. if you do put it in the loft then according to the gas regs you need a proper loft ladder installed for ease of access and a boarded walkway from the loft hatch to the boiler with a safety hand rail.
Thirdly, if your going for a combi or any boiler for that matter look no further than a Vaillant or a Worcester-Bosch Greenstar, no point in considering any other make in my opinion, a bit more pricey but well worth it.