davidjohnson6,thank you.
You are the one who really understand what is the issue in my case. In accordance to UK immigration rules I can enter UK without visa, but EasyJet airport representatives in some countries are not very familiar with UK-TWOV rules. There are a lot of stories in Russian forums when people were denied from flying to UK without visas by EasyJet airport representatives (mostly on U2 route ex HEL to LGW).
I wanted to protect myself and requested official reply from EasyJet online customer help to print it and show in airport, but received from 2 persons there completely opposite answers.
Typical reply from UK Embassy for discussing question is:
"Yes, you can travel without visa, but it is better to apply for transit visa anyway." What do the mean saying "better"?! It is absolutely not juridical term for me.