Right then for ground floor at a temp of 15 d
work shop with 2 out side walls needs 3300 btu`s
toilet with 2 out side walls needs 1550 btu`s
garage with 3 out side walls needs 14000 btu`s

upstairs with a temp of 21 d
with the roof and only slight walls befor roof about 23000 btu`s

total heat needed is 41850 btu`s plus pipe work ect so grand total is 45000 btu`s but you will need to sort the caps out around garage door and also stick celotex insulation board to inside garage door to help heat loss in garage

so you are looking for a boiler that has 15kw out put, the biggest electric boiler is 12 kw this whould do it but it whould take longer to heat up but it whould be worth trying to see if its copes ok if not you could allways add a second smaller electric boiler to the system to up the out put, or put in an oil boiler ect.
hope this helps dunc