JRM has said that (source, his Moggcast vlog, per the Times), "No sensible party or government ever breaks manifesto commitments willy nilly: it needs extraordinary circumstances..."
Said a letter in the Times today: "I can't think of any circumstances that might be more extraordinary."
I don't see that the Government have a choice. Taxes will have to increase at some point. Under the circumstances I don't see that tax increases alone would be so electorally devastating.