Unfortunatley flogas are pretty hopeless - I've got one of their key accounts for the ex-countrywide pumps, and getting info out of them is like getting blood out of a stone at the best of times.
www.Filllpg.co.uk might be your best bet to locate where some might be found around you.
If you do go for using bottles you want to secure the bottle somehow just to be on the safe side (given the lack of a shutoff in the event of something happening to pull the outlet hose off the bottle).
Calor prices will probabbly make using petrol nearly the same given the current price (102.7 here at Asda vs 59.9 for gas on the pump). Flogas were somewhat better though still were not as cheap as elsewhere, was working out about 84p a litre (without taking any cylinder deposit into account). Calor works out around £1.34 a litre (based on 19kg cylinder prices, £32 from flogas, £51 from Calor).
If you can find a local supplier (example in your area is
https://www.fourseasonsfuel.co.uk/ ) you might get a better price. I buy the same cylinders as above from Birmingham autogas (for use on a space heater) at £17 from them.