thank you. The starting point is there is no right of cancellation in respect of contracts for lettings so once you make a contract it is binding on the parties unless the parties agree to rescind ( cancel) the contract or if you can show grounds for rescinding the contract unilaterally.
from what you describe, the only grounds you may be able to show would be that you understood that you would have exclusive occupation of the property during your occupation but after making the contract, the landlord sought to introduce a term allowing for access for viewings and so on. You can construct an argument that this is a fundamental change to the basis of the agreement you understood you are entering into and as such, is grounds for recession.
the effectiveness of your argument will turn in no small part upon any evidence there is for or to the contrary that the term for third party viewings was introduced by the landlord after you made your agreement.
in terms of recovering your monies,, you would initially need to set out the basis for your claim for a refund as above and then, if you paid by credit or debit card, if the landlord refuses too refund your deposit, you could seek a charge back process whereby your card issuer disputes the charge on the above grounds or if you pay by bank transfer, may necessitate a claim in the County Court. if the landlord can demonstrate that on balance you were aware of this condition for access, then this would severely prejudice the success of any potential claim: