Yes, IMO the fault is entirely yours. You should have specified that all ofcuts are to be delivered. Also the delivery fee would have been quoted before you paid for it, so you had the option of declineing it and going to Mitre 10 or anywhere else.
Then you sent an email complaining about it. Question is did you make email the normal way of communicating with them by placing your order by email. If not, why do you think they have someone sitting at a computer waiting for emails. For small businesses emails are not the normal way for probably they are only checked once a day ( if you're lucky) and the phone is the normal method of communicating.
But more over you publicly named them on a world wide read forum because of your mistake in not being specific or using the normal way of communicating.
Then you published their reply that was very reasonable unlike you.
By all means take it to the department of fair trading, then it will escalate further with lawyers at twenty paces and you will lose and pay everybodies legal and other expenses. Stupid I know but you would probably want to do that just to find out that you were in the wrong and not the supplier.
Peter.