Difficulty with real estate agencyin Italy

It sounds really unusual.
The " preliminare " is carrying the buying promise of the buyer and the  selling promise of the real estate owner. No way that this is a kind of deed signed by the seller and the buyer, and no preclusion about his content may rise among the parties. There is no secrets or other reason that clauses or any other things would not be known before the signing of this document.

This is bizarre on many levels.
Firstly you accepted a proposta di acquisto that presumably had a scadenza.   you say you heard noothing for months -  if the proposta was scaduto, then the agency should have sent you the caparra paid.   The buyer decided to go for a mortgage - it is something the agency should have discussed with you, as you are not bound to accept that as a term of purchase and it should have been written into the proposta.
You do not, technically, need a procura for a compromesso - though it is desirable.  The agency cannot deny you a copy of the compromesso - it is a carta privata and you are a signatory even by procura.  The notary could askfor the id of the lawyer, presumably the compromesso will be signed at a notary?  I would contact the notary direct.  The agency works for both buyer and seller, and to me it sounds a little suspiciious that they are treating you in this way.  If you are givinga procura to a UK based lawyer, will he go to Italy to sign the compromesso? or are you using the lawyer merely to draft a procura to a third party?

Many thanks to both of you for responding - we too found this completely unreasonable and bizarre.
Modicasa: the proposta did have a scadenza that expired but we never received the down payment, agency then announced owner was going for a mortgage which we weren't happy about but accepted (though of course it hadn't been written in the proposta).
Contacting the notary directly is a good idea in theory but the agency says that it is actually the notary who is insisting on seeing lawyer's ID before we can see the preliminary contract. Even if that were true, the agency obviously should not accept such an absurd demand but I think pursuing communication with this particular agency rep is fruitless and we'll need to contact a head office.
As you say the agency works for both buyer and seller and this is the fundamental difference between Italy and the UK where only the seller pays a commission.
I assume that asking for our lawyer's ID is legitimate in itself but to demand it before showing us a draft of the compromesso seems really dodgy. We wouldn't go to Italy to sign - I believe the lawyer would do the procura process via the consulate, which would then need an apostille.

I think the agency has fundamentally failed you in due diligence.   At the moment you have no agreement to sell the house, and the agency failed to inform you of information you had a legal right to know. 
Once the proposta was scaduta, the agency has a legal obligation to give you the caparra paid out by the buyer (unless you waive it).    You say the 'rep' is a bit useless - a rep is no good - you should have a qualified estate agent working for you and if the agent is using a bloke 'who's keen to learn' you can refuse it.   Unfortunately some of the 'luxury' foreign agents have little knowledge of Italian law.  I had to work with one last year which was, frankly, embarassing - especially as they ask 4% - which can amount to alot of money on a luxury property. 
Anyway - yes, the notary can ask for what he likes, but I doubt you are getting the full picture from the agency which will be telling you on a 'need to know' basis.   I would put your position in writing to the agency, telling them that a you are unhappy with their work, and if they want their commission they need to work on your behalf.  You have every right to see and check the preliminary of sale with lawyers, accountants - anyone you fancy.   I routinely send compromessi to all involved for checks and tweaks before the signing - its obvious that it is a legal agreement and needs to be acceptable to all parties.   However, Italain notaries will probably only prepare the text the morning of the signing, so it can be difficult - but insist.    If you need more info then pm me.

Thanks Modicasa, this is very helpful, I've written to the agency as you suggested.