The pressure won't come from Labour, it will come from Tory backbenchers before anything becomes HMG policy and hence part of a FB. It depends how strongly they feel about it. Leverage isn't applied vote-by-vote, it's about deals, and there are many other issues on which backbenchers have a decisive vote. If they were just going to do it, they wouldn't be briefing it out three weeks before a budget. They'd just do it in the budget. It's standard trial balloon MO - HMT/HMRC clearly want to do this (and had planned it from the start), but they also want to gauge pushback from business and the backbenches.