IMHO You are being royally shafted. The original quote was IMHO about 50% higher than what it should have been if you were supplying all the kit. (OK I haven't seen it and its a guess) As to the extra work! As others have said it should be a fraction of the numbers being quoted. Even moving the gas pipe is a mornings work for a gas safe person - £200 and a bit of material at the most, and prob less. If you are working on the floor, it is a thermal element under regs, and if more than 50% is altered, a building regs application should be made. (prob hence why he has stopped on a small area)
So, step 1. Tell the builder to stop. Say you have decided that for environmental reasons you want insulation in the floor which means the whole floor has to come up and you need to make a regs application, so the work is stalled until a specification is prepared and an application made.
Step 2. I'm guessing there is no contract with formal termination clauses. If this is the case, write to the builder formally telling him for the above reason you want to cease the relationship and terminate the contract. Say as there is no formal contract, under the principal of quantum meruit (look it up) he is entitled to be paid for the work done to date. Say that according to your records he has been on site for x days and therefore is entitled to x times the going day rate (be generous say £350 a day outside london, perhaps £450 in london.) This is a generous rate that should be difficult to dispute. Say you will also add on £500 for stuff he has used like tools and sundry materials (or ask for itemised bill of materials used so far), and then get rid of him. Hopefully he hasn't been there for 6 weeks just drinking brews. If he has you'll have to estimate how many days of real work have been done....
If you make a fair offer for work done, it will be difficult to dispute. He might try it on with loss of profit etc. but unless these terms are explicitly stated in termination clauses in a formal contract, you can ignore. If he kicks back suggest that you get an independent quantity surveyor to properly assess the work done and value it (unlikely this will be needed). If he says he has bought materials, only agree to pay for bespoke materials. Ordinary stuff is usually returnable or reusable, and again you can fall back on the lack of contractual terms that should state what happens to payment for materials still off site. (usually fair payment is made for goods delivered and work done). The lack of contract and payment terms works in your favour.
Basically get rid and start again. You will save far more than the cost of buying your way out of this relationship. Just be contractual, not confrontational. If you know anyone involved with construction contracts, bring them in as added support if you can.
Once you have cleared the air you can decide if you really do want to add insulation to the whole floor or just patch up what has been done, but the absence of a real DPM would concern me...
Watch last week's DIY SOS... I suspect with the kitchen still to fit there will be more "extras" on the horizon......