Sorry I can't take a call. When you speak to the manager today, tell him there has been a change in contract, not agreed by yourself - therefore they are in breach of contract.
If you booked by credit card then you can make a claim - you can contact the card provider and ask for a refund under S.75 of the Consumer Credit Act 1974 - as they are jointly liable if you do not get the service you paid for. If you did not book with a credit card (or you did and the card company refuses to reimburse) then you should consider suing the venue.
The venue should not be charging you for a service which they cannot offer. The essential term to the contract you have with them is that you pay them - and in return they provide the venue for your wedding. The fact they moved the goal posts in a contract and now refuse to refund you is likely to mean they are in breach of contract - meaning you can terminate and demand your money back. A court will take a dim view if it looks like the venue penalises you financially through no fault of your own.
The term here is "force majeure", or an "act of God" clause - meaning that if performance of the contract is impossible then the parties can agree to suspend their obligations - for the duration of the force majeure, or in this case, for as long as the service cannot be carried out due to the lock down. If they simply change the terms and you do not agree, the contract is terminated.
If they dispute this, threaten them with a report to Trading Standards (tel: 03454 040506). You will need to send them a letter before action (see template attached) and give them 30 days to pay you. If they do not pay you, start a claim via the Money Claims website: http://www.moneyclaim.gov.uk
I hope this helps? If you would kindly leave me a 5 star rating (at the top of your screen), the question will stay open and any follow up questions are welcomed.