I am not a lawyer. But I do know that a contract doesn't over ride actual law. For example, standard contracts usually have a clause about allowing access to LL for viewings at end of tenancy. There is no actual law to enforce that contract clause and a tenant who signs that contract still isn't legally obligated to allow anyone in. In fact, in order to obtain emergency repair access, a landlord needs to first obtain a court order. He won't get one for viewings.
First, contractual right is "actual law". So, if you enter a contract agreeing to something, the starting point is that that thing is fine (subject to a few exceptions such as exclusions under the Unfair Contract Terms Act).
I think what you may have been trying to say is that even if a tenant has signed a contract with a clause allowing the landlord access for viewings, the landlord would not be able to enforce that right as it would effectively be "overruled" by the tenant's right to quiet enjoyment of the property.
The tenant does have a right to quiet enjoyment, but there is no common law rule or statute which says the right to quiet enjoyment absolutely overrules a contractual right to access. The tenant's right is to enjoy the property unharassed. If the landlord accesses the property on reasonable notice at reasonable times of the day for a purpose the tenant has contractually consented to, that is fine. Clauses permitting access for viewings usually say on 24 hours' notice anyway, as do clauses permitting access for repairs. Also, clauses permitting access for viewings tend to be limited to the last month or two of the tenancy, which also balances the tenant's right to quiet enjoyment.
Next the question is: so what if you do object and the landlord can't access for viewings as per his contractual right? His damages will be the measure of the loss he has sustained as a result of you not allowing the viewing as you are contractually bound to do. So if he had a potential tenant willing to say "I would definitely have rented that property for one year at £2k a month, but as I couldn't see it, i rented another", then the damages would be around £24k.
Conversely, if you refuse access but the landlord does access anyway, then you would be likely to lose any claim against the landlord because you had already consented to reasonable access for viewings in the contract. However, even if you did win, the measure of damages would be your loss as a result of the viewing, which would be... What, exactly? If you sustain no loss, then you aren't entitled to any damages. So if the property is left as it was before the viewing, you have no real claim.
In practice of course it's unlikely people will make claims over something like this. It's better to cooperate with the landlord and come to some agreement about viewings. But if you really hate your landlord and decide to dig your heels in, you could be in breach of contract (if your contract contains a clause permitting access for viewings) and that is more likely to cause the landlord a recoverable loss than his accessing the property is likely to cause you a recoverable loss.