It depends whether the property is registered with the Land Registry or not. If it is - and most properties are - then the deeds are of no real significance as the Land Registry will have recorded etails of everything that concerns the property. If you need a copy of the Land Registry entry this can be got on payment of a fee.
If the property is not registered then the title deeds are important - if they are lost or destroyed you would need to swear a declaration explaining what has happened to them. An application would then need to be made to the Land registry for the title to be registered and the LR will check to see if any other claims have been made in respect of the property. It can get a bit messy so you would need to see a solicitor if you have lost them