You can instruct solicitors before you have a buyer - in fact, it makes everyone's life easier & the transaction run smoother if you do.
You can get all the forms (Seller's Property Information Form, Fittings & Contents form, leasehold information form) filled in & ready to go in advance & then it give you time to get copies of any documents that have got lost along the way & which hold up transactions.
For example, make sure you have copies of all the planning consents (and those from before you purchased - check your local council's website), building control approvals, FENSA certificates guarantees for replacement windows, re-wiring, boiler installations etc.
It's those sort of things which usually hold up transactions - not weighty & complex legal issues. Your lawyer will love you if you get all that sorted in advance - well, I'd love you, if you were my client!
Oh, and if the estate agent is recommending someone - they've be getting 1/3 - 1/2 of the fee you pay the lawyer, for doing zero work.