James good luck, I'm sure you build fine amps.
As others have said, you face some pretty stiff competition. You will need some good marketing, I've just had a look and there are 2 other UK builders offering 4xEL84 hand built heads for less so you would need to explain why folks would shell out a little more on your amp?
I hope you succeed but risking financial ruin on a product more expensive than the competition when you have not yet built up a brand name might be a little risky.
But all the best, you could be the next Dumble and I'll wonder why I didn't buy one of your amps for a mere £850!!