OK, so I went through the chipping today and I actually got a chance to speak to the AVID representative and I wanted to let everyone know what happened...
So, the Avid EURO chips are NOT 134.2 kHz like I thought - they are 125kHz. They comply with the standards set up by "FECAVA" and so are acceptable for travel to the UK and the EU.
The rep said that yes, there are vets in the UK that will be able to read it BUT there may be some that can't so I might want to rechip them all with 134.2 kHz in the UK.
So, like others have mentioned - HomeAgain is safe for traveling, so is Avid Euro Chip but the best bet is the 134.2 kHz chips (ie the ResQ chips available in the US) for being read in Europe if you're going to live there.
Another tidbit: I did not double chip my three kitties that already had HomeAgain - because their chips will be Ok for traveling but also because the rep says that two chips of similar frequencies can interfere with each other. Just for general knowledge.
So, I ended up chipping my one baby who didn't have anything with an AVID euro chip that should be fine for getting into the UK (just like the other babies with their "Home Again" chips) BUT may not be Ok for living in the UK.
SIGH!