Tax rules are there to incentivise businesses to invest. As...

Tax rules are there to incentivise businesses to invest. As companies have invested on this basis, have spent money in prior years its only logical the government will phase out the tax advantage so those who spent on the current system are treated fairly by accessing the tax advantages and can complete their projects viably.
new explorers would have to then run their numbers on the new system to decide if it is a wise investment.

this is the logical outcome. The Labour Party get to sell their green policy - close their "loophole" - without upsetting business groups and 200,000 oil and gas workers. The fine details would be agreed before the election to stop oil and gas from campaigning against them during the election. Voters would only find out about the walk back after the election when the final bill passes.

this would be the optimum win win scenario in my opinion. Labour are only after the win after all, they are not blind idealists.