Here is a very brief summary of the process by which somebody could (in theory only) sell their own house:
1. Prepare the contract for sale. You might be able to do this yourself, but I doubt it. Specifically, your title documents will not be released direct to you by your lender; they will release these to a solicitor on loan (on Accountable Trust as it is called) for the obvious reason that a solicitor is bonded and can be held acountable for loss of the deeds. Conceivably, you could prepare contracts based on getting a photocopy only of your title documents, but that still leaves the problem of having the necessary expertise.
2. Reply to the "requisitions on title" - i.e. a set of questions raised by the solicitor for the purchaser. You might be able to do this yourself, but again I doubt it.
3. Prepare some of the closing documents; approve some closing documents prepared by the purchasers solicitor. You might be able to push this job entirely onto the purchaser's solicitor, but I doubt it. And anyway, you wouldn't really know whether what you are signing is appropriate.
4. Close the sale; This would be particularly tricky. Specifically, no solicitor will accept your undertaking to discharge the mortgage out of the sale proceeds, or indeed to hand over the title documents (held by your lender) at some later date. So you would have to complete the sale at the office of the bank\building society who hold the mortgage and they would have to hand over the documents and give confirmation on the spot that the mortgage has been fully redeemed AND that they will send the vacated\released mortgage to the buyer's solicitor (because he will not accept your undertaking to forward it)
I think a previous contributor summarised the matter best, when he\she compared it to trying to carry out your own plastic surgery.
Good luck if you decide to go ahead with it.