Your website T&Cs include this clause: "If a provision of these terms and conditions is determined by any court or other competent authority to be unlawful and/or unenforceable, the other provisions will continue in effect. If any unlawful and/or unenforceable provision of these terms and conditions would be lawful or enforceable if part of it were deleted, that part will be deemed to be deleted, and the rest of the provision will continue in effect."
What is the function of this clause?
The law does not allow complete freedom to contract. Contractual clauses may be ineffective or even unlawful. To take an extreme example, a contract to commit a crime will not be enforceable on public policy grounds.
Consumer protection law, competition law and the law relating to exclusion clauses (eg the Unfair Contract Terms Act 1977) will prevent the inclusion of certain types of clauses in a contract. However, the law is not always 100% clear, and sometimes it is desirable to include a clause which might in fact be unenforceable.
Severability clauses are often included in contracts in an attempt to anticipate the issues that arise when a provision is found to be unenforceable or is removed from the contract on other grounds. They attempt to define the basis upon which offending provisions may be excised from the contract.
Usually, the parties will want a contract to continue, particularly where the unenforceable clause is not central to the contract.
However, severability clauses should be treated with some caution.
Judges don't like being told what to do, and they may consider that the consequences of a clause being removed from contract are matters for them.