If you need to be away when your house is for sale, or if you become incapacitated or physically unable to sign paperwork, it is possible to sign by way of a power of attorney.
If you have an existing Continuing Power of Attorney for Property, this can be used to sell your home. If not, you can either create a new one, or sign a Limited Power of Attorney for Property that deals just with the specific property you are selling.
Either way, the power of attorney is registered in the local Land Registry Office so that it can be referenced on the sale document. You will need the original power of attorney; a copy will not do. Your lawyer will also likely want to talk to you, or to a doctor if you are incapable, to be sure that it is being used appropriately.
If done properly, powers of attorney can make a sale go much more smoothly in difficult situations.