As you have withdrawn consent for the web designer to access the domain, then it is indeed a breach of the Computer Misuse Act. That is a criminal offence but the police will not be interested in all honesty because of the earlier relationship between you and the web designer clouding the issue. However, you should refer the matter to the police anyway and get a crime number to refer to 123-reg (who in my experience of always been extremely helpful) and also to refer to the web designer.
You can also threaten the web designer with an application to court for an injunction firstly to making give you the access details again and secondly to prevent him breaching the Consumer Misuse Act again. That threat will have more weight coming from a solicitor.
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