I am an English born Australian citizen, travelling on an Australian passport. I have an English passport which has been expired for several months. I am going to England to visit family next month. Will i have any problem, not having a visa.

If you are an Australian citizen you don't need a visa to visit the UK for less than six months, but as with anybody presenting themselves to a border should be prepared to show evidence that the trip is genuine, you can support yourself and leave the UK at the conclusion of the trip.

You could also enter the UK on your expired UK passport, a passport is only evidence of your citizenship and your right to enter the UK unhindered, and whilst the document might expire, your citizenship doesn't.

You might have a problem using an expired passport to travel though.

I will also visit the UK soon and have 2 passports, one UK and one NZ. Neither expired.

I arrived in Thailand on my NZ passport and will leave on the NZ passport. (with re-entry permit)

I intend to arrive and depart the UK on my UK passport.

I will then re-enter Thailand on my NZ passport.

Don't see I will have any problem with that.

Dual nationals do that all the time; use the passport most convenient.

As long as you use the same passport to leave a country as you used to enter it you wont have a problem.