I hate to say it but your calculations are not correct Ethel. You need to remove 17.5% VAT and then add 15% vat, therefore, the new charge for the same item should be 97.5p (typed as 97.8 in my question in error.... and I have found the answer now for Poundland....
The price of �1 included VAT at 17.5%.
The price net of any VAT is �1/1.175= �0.851
The new price should be �0.851 x 1.15= �0.979
So you could argue Poundland should reduce prices by 2.1p.
But of course that's not the point of poundland- their prices stay the same whatever inflation and taxes are.
For existing stock there's an argument for saying Poundland should give a 2% discount off the final bill at the till. But they can sell goods for whatever price they like and I'm sure they'll argue their prices have stayed the same for many years so it wouldn't be unreasonable to put prices up by 2p now. And as they get new stock in, and maybe change their products they can charge what they like.
Moving away from Poundland, I bet the cost of many things won't come down in practice tomorrow.