I'll start by appologising for asking this question which must have been asked many times before but for the life of me I cannot find the information I need!

My wife (Thai) and I are hoping to visit UK in July (I am a British citizen BTW) and I'm sure I've read before that one cannot apply until 3 months before travel. Can anyone verify this? Thanks.

You can ask for the visa to post dated by up to 3 months.

For our last years visit in June we started the application towards the end of March and asked for the visa to start 1st June. If you don't ask for a specific start date they date it from when it is processed.

(Just a point, don't assume that they will date it from your proposed travel date as stated on the application form. I know from experience that the ECO does not draw logical conclusions. We put the request in the additional information box at the end of the application so that it is officially recorded , I put it in my letter and I stapled the request to the front of the submitted hard copy so they couldn't miss it.)

In addition to the excellent advice from rawhod above, when she gets her passport back check the start date immediately.

If they do still get it wrong; the longer you leave it before notifying them, the harder it will be to rectify the mistake.