Ummmmmmmmm. Not a good situation.
The best possible scenario is that you approach builder and ask to be released from contract with a full refund of deposit paid. Depending on, as you say,"current market" and whether or not he is a nice person, this could work. Or not.
"Can we sell the property before completion? We are both FTBs, what are the FTB & stamp implications for this?"
Probably not. You would almost certainly need Builders consent which he might give. Is there a market for the property?
"Should we wait for completion and then put it on the market?"
Probably not, if you can get out of it beforehand.
Should our (joint) solicitor be able to advise on this?
Yes - but s/he will remind you that you are contractually bound to the contract and that you will be doing well to walk away without it costing.
mf