Can implied terms be excluded from a tenancy agreement? (UK)
The exclusion of terms that are implied by statutes is null and void. A contract is not supposed to override or contravene legislation or public policy. Likewise, it would be futile for a contract to single-handedly "establish" that certain statute does not apply to that contract (unless legislation itself reflects that the statute is somehow optional).
The specific example of some statute providing that "[T]he landlord must carry out basic repairs with no cost to the tenant" appears intended to protect tenants against the greater bargain power that a landlord typically has. Thus, the enforcement of a contractual exclusion of that statute would defeat legislative intent, in which case the statute is not cognizable at law.
I am wondering if it is legal to exclude implied terms in a tenancy agreement and whether I should question/amend the lease before I sign it?
The legality depends on the statutes involved. Some statutes not only nullify --be it by implication or explicitly-- the effect of clauses that contravene legislation, but also outlaw even the mere existence of such clauses.
There is no general answer on the need for an amendment of a lease that excludes implied terms. The detailed terms of the lease and of the legislation might be such that an amendment is unnecessary because the clause is null and void in the first place.