speed increases happen from time to time but they do not usually incur a cost increase - if you bill has gone up because of that then afaik thats wrong - on the other hand they do have annual price increases - there is one in progress - if thats what been added to your bill then that would have happened had your speed not increased
you need to talk to them to clarify whats happened - try this
dial 150 or 0345 454 1111 and follow the prompts to - 'thinking of leaving us' - then "technical issues" - dont worry about that they do much more - open 8am -10pm 7 days a week - should take you to a UK call centre
they may be able to offer you a better deal - if you are out of contract and want to leave then you need to give 30 days notice - if you are in contract and the increase is due to an annual price increase then you can leave - again with 30 days notice with no penalty
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Tony