You will be bound by the clauses (ie. terms and conditions) as long as you formally executed the agreement unless any of the terms are deemed "unfair" under the Unfair Terms in Consumer Contracts Regulations (UTCCR). However, you do not really want to be in a position whereby you are forced to rely on UTCCR.
There is a possibility that you could use the Distance Selling Regulations 2000, but if you are able to terminate the agreement in accordance with the contractual terms then this is the safest way of getting out of the agreement.
When did you sign the terms and condition?
You should check the terms of conditions for the procedure to terminate during the 7 day period to see whether this entitles you to a full refund of the money you have paid. If it does and you consider you doubts persuasive enough so that you wish to terminate the agreement then you should immediately terminate in the manner specified in the contract.
If it says it can be done by post then (because of the tight timescale) you should serve your notice to terminate on them ideally by physically to them at the address specified. Get a receipt if possible. If not you could consider using Royal Mail's Next Day Special Delivery service to ensure it gets there tomorrow. But again, you should calculate the expiry of the 7 day period and check you are in time (the contract should state when the 7 day period is to run from).
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