While our construction costs are indeed ridiculous, this number is incorrect. It hard to decipher which £1.2bn figure you are actually talking about - but none of them are for just planning (for reference: the contract for the northern connecting highways and the contract for the actual tunnelling are both for a similar amount of money; the total spend as of 2025 is also around the same amount but it includes initial payments on all contracts etc).
I got the 1.2B from the Beeb: https://www.bbc.co.uk/news/articles/c0mrm84wg4ko.amp
> Plans for the 14.5-mile (23km) route were approved in March after a 16-year process that has already cost £1.2bn.