you need to ring retentions and see what they can offer
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
if you are out of contract you could give 30 days notice - that usually prompts a call from outbound retentions with a good offer - if they do ring its usually take it or leave it - if you leave it and ring back the 'normal' retentions will not be able to match it
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Tony