Does that constitute a contract?

That was an offer. A contract was created when you acceepted their
offer, unless they'd withdrawn or amended it between offer &
acceptance. See their terms and conditions for something like "...valid
for 30 days...". . They may have quoted their current list prices and
the prices had increased before you had placed the order.

What offence has ben committed?
Who's Ben? No offence that I'm aware of, they're in breach (or is that
breech?) of the contract. Ask on Uk.legal.