When does an AST become legally binding?

We are letting our flat and buying somewhere new. We have found tenants and happy to proceed but have not exchanged on new place. They are keen to sign and I am keen to sign them too (keep the momentum going) but at what point does the AST become legally binding? Does money need to change hands (the old consideration bit). Is it when they pay the deposit or first month's rent? If we both signed and we don't exchange because our purchase has gone pear-shaped, what recourse do they have? We couldn't move out prior to exchange/completion - too risky.

I have a few stalling mechanisms so may just try to string it out a bit (get guarantors to sign etc. etc.) until exchange.

Also, sorry, further quick question, do we have to meet face to face or can the whole signing/witnessing bit be done through the post - so they sign two copies of the AST (witnessed), send back to me, I sign both (witnessed) and send one back to them?

Any input gratefully received. Many thanks

Yellcom