Best plug in heater?

[nominous]

I have an air conditioner but sadly it doesn't do heating.

Bugger. Can mod them, but it's something worth trying out on an old fridge for the first time

I'm not sure I have enough garden for a decent GSHP system but do have an ideal south facing location for both that and solar.

On my list too. Was planning to go down. Watched a ton of vids and read loads on DIY boreholes and spoke to a couple of folks. It's surprisingly easy.
Plus going down with enough solar thermal and a good GHSP you can dump the excess summer heat into the ground, thus raising the temp and making it more efficient in the winter.

Loft insulation will be done soon. I keep meaning to do it, there is some in there on the loft floor rather than the roof but I keep being reminded that a house looses a third of its heat through the roof.

The floor and roof are both worth a hit.
Unless you want a very cold loft in winter and a very warm one in summer it's worthwhile insulating the roof itself too. Especially if you keep stuff up there.
The suggested levels of insulation are generally incompatible with a desire to store anything and board it out. Unless you've got a new build with fecking matchstick joists going everywhere anyway.