Builder Taking Too Long or the Price Went Up

If the price was not agreed you only have to pay a reasonable price (s.51), and if no time was agreed the work must be done within a reasonable time (s.52). source

Services must be performed with reasonable care and skill (Consumer Rights Act 2015); if not, you can ask for the work to be redone or a price reduction, and go straight to a price reduction if redoing is impossible, takes too long or causes significant inconvenience. source

You have up to 6 years in England, Wales and NI (5 in Scotland) to report problems with services, even if not immediately obvious. source

TrustMark registered businesses are vetted and monitored by scheme providers and are expected to give a minimum 2-year guarantee. Their insurance must cover the business ceasing to trade, including workmanship and putting defects right. Unresolved disputes can go to the Dispute Resolution Ombudsman, and a 'Trusted Payments' option releases your money in milestones, only as you sign off the work. source

Section 75: if you paid part or all by credit card for an item costing over 100 pounds and up to 30,000 pounds you can claim against the card provider for breach of contract, but you usually cannot use it if you did not buy directly from the trader. source