the cheepest way whould be to come off your house heating and put rads in your garage, i whould put them on a new zone so you can easy switch them on and off by room stat and/or time switch. pipes whould need to be 750mm below ground level and well insulated, your boiler you have now should cope ok with this because of the sperate heating zone.
Or you could put in an electric boiler, cost about £350 for a 2k up to about £550 for 12 kw, you whould need pump and unvented kit aswell which will cost about £100 on top, these are very small boilers 550mm H 164mm W 130 D and the biggest one is just longer at 1073mm H quite cheep to run and alot cheeper than an oil boiler, plus cheeper fitting, just depends on electric supply to garage, but if you only want say back ground heat the 6kw whould do ok and whould run 3 1200mm x 600 mm doulde convector radiators if needed.
ok hope i have given you soom ideas cheers dunc

ps iam a plumber / heat eng