If they do decide to tax gratuities, they may well find that their tax take isn't quite as much as they anticipate if such a measure is extended to all gratuities rather than those earning say over 100K.
Many people commute their pensions precisely because it is tax efficient to do so. Get a bigger chunk of your pensions up front tax free and also lower your annual tax bill up to 55 by way of lower annual pension payments. But if you make commutation an unattractive option, rather than getting 40% of an 80K gratuity, people will just simply not commute and HMT will instead be looking at 40% of 40K (very rough figures).
Then if an individual has any fiscal nouse, they will be looking at ways to legitimately minimise their tax burden, either by putting whatever money into a tax free ISA or a private pension. If they go for the pension route, then the government will have to stump up 20% tax relief on your contributions, which on a 10K / yr pension contribution would amount to 2K tax relief, which if extrapolated over the years means that you would probably (eventually!) get back in tax relief what you had lost in your gratuity being taxed. If they go down the ISA route, then whilst there is no tax relief up front, neither is there any tax to pay downstream - yet more money safe from HMT's grubby little mits.
I admit that squirelling away your immediate pension for the future means you can't spend it now - and you would have to ensure that in doing so you could maintain your commitments and standard of living - but the point I am getting at is that HMT, whether they intended it or not, have started a game of cat and mouse. For every hairbrained scheme HMT comes up with, any individual with a degree of nouse about them will look for ways round it. I'm fairly sure that as part of George's 'Plan A' , a discussion was had that went along the lines of lets keep interest rates low to encourage domestic consumption. But as tax rates and inflation went up, all it did was to make it more attractive to pay down debt and over pay on the mortgage. If they do decide to tax gratuities, the laws of unitended consequences will simply kick in once more. Unfortunately for Dave and George, many people are now starting to wake up to the fact of what the Government is doing and are actively taking steps within the regulations to preserve their own financial interests.