Not surprising. The budget was so written that it was bound to annoy. When we have a chancellor cutting the top rate of income tax back to 40 per cent, thereby making a lot of high earners £40,000 a year in tax saving, we have a chancellor who has not exactly got the national mood, whatever the economics behind that decision. And now we have this strange idea that giving to charities which are not, in fact, charitable but simply a means of getting the money back in expenses etc of the donor, is a serious tax dodge. This idea is not being backed by any figures as to the frequency or rarity of this devious scheme. And anyway, all registered charities in the UK have to show charitable purpose and have their accounts checked for the Charity Commissioners (if that's their current name!), to show that their own expenses are appropriate and their charitable purpose is being served.
Couple that with the usual 'mid-term' unpopularity of a government and it's surprising that the poll lead isn't greater, though that may be a reflection on the Leader of the Opposition, who does not appear to be very effective.