Using multiple passports
 

I will be leaving AU on my AU passport and enter England in transit to Rome.

I assume I will need to clear immigration in London and intend to use my Italian passport.

Now I come back to London by rail and again will enter using my Italian Passport. I then leave for the US and assume that it will be best for me to use the same passport leaving London and entering US.

Will either be okay or is it best to use my AU as I will later be leaving for AU from LAX and returning to Brisbane where I will need to use my AU passport.

Strictly speaking you're supposed to start and end flights on the same passport. In practice I haven't done that a few times (I travel with both, often forget which I check in with) and I've never had trouble. Not to say you WON'T have trouble though ;)

Strictly speaking you are supposed to choose a title for your a new thread reflecting what it is about.

To add something constructive: I have shown a different passport from the one I've used to check in as well and had no problems. I've also shown both passports at check-in without problems.

HTB.

ah, but the ambiguous titles never fail to draw lots of attention.

There is no outbound Immigration control in the UK so the only people to whom you would show your passport on departure would be airline staff at check-in and at the gate. They will record your details for US Immigration so you should show them whatever passport you intend to use to enter the USA. They won't know what passport you used to enter the UK so that doesn't come into the equation.

Needless to say you want to use an EU passport to enter the UK.

for the 5 years i was living in the EU, it always seemed to make the most sense to travel on the passport of the place i was arriving... so when going to the EU I would use my irish passport and when going to the USA i would use my US passport. The one time I goofed and used both in one journey it caused confusion and I ended up having to produce the other one anyway.

definitely use your AU one when you leave the USA, regardless of which you use to enter the USA. A resident going home has fewer hoops to jump through.

I travel with both of mine and have forgotten which one to use many times. It's a crap shoot whether you'll run into issues.