I pay 17p per kWh, the joys of living in Northern Ireland!
Sonny - Most moden multisplit systems will reach a CoP of 4 - Dropping to about 3.4 in very cold weather. It has nothing to do with the size of, or thermal performance of your house.
I'm talking about your initial statement - that heat pumps need days to warm up. Which is entirely wrong. Plenty of people have posted to say that it takes about 30 minutes to warm the house - I design these things as it's my job.
Look to something like Panasonic, Daikin, Mitsubishi, for quality stuff.
I'm now trying out a Daikin unit (seem to be the test bed for our competition) which is sorta a hassle having the installers in, but after the year is out they pay for the place (my house) house to be painted by "professional" painters, so that's a bonus.
It's been repainted 4 times in 7 years, and if I'm bothered then doing all white I can get my own colours, the painters do great, and I still get it free.