I guess you've made up your mind to go for GSHP, but on other posts about this there are interesting suggestions on what you could achieve by investing the cost of GSHP into making your house passive. You are already fitting HRV and you may find that your heat requirements would never justify the capital cost of GSHP.
It is ironic that many of the houses installing state of the art heating systems require very little heat, while the breeze-block monstrosities are using oil.
Even if you don't go passive, you may find that a very small pellet or wood stove would meet your needs.
But assuming you have made up your mind, regardless of who you ruin with, GSHP with UFH has a slow enough response time and works on the basis of keeping your house warm all day long while you are out at work. You often get another top-up in the evening to keep the house warm for the night. That top-up happens at a time of peak consumption when wholesale electricity prices can peak at 25c. When smart metering penalises this time slot with a retail price of 40c or 50c, it will be problematic. I would consider a large buffer tank which ensured that all of the heat requirements can happen off peak at night.