Actually, doesn't look like that term applies in this case:

4.1 We may vary any of the terms of your agreement, including our Price Plans, on the following basis:

(a) any updated Price Plans and new terms will be available on our website and on request to 3 Customer Services;

(b) if you are a Pay Monthly Customer, we will let you know at least one month in advance if we decide to:

(i) discontinue your Price Plan; or

(ii) make any variations to your agreement which are likely to be of detriment to you; or

(iii) increase the fixed periodic charges for your Price Plan (if applicable) by an amount which is more than the percentage increase in the Retail Prices Index Figure (or any future equivalent) in any twelve month period.

You can end the agreement for such variations as explained in Section 10.

The rate is not higher than RPI, so Three is arguing that you have no right to cancel. I think that's terrible.