Interrail IS valid in GB. The RDG press release is outdated and should be removed. RDG's decision was reversed one day after its publication. You could tell Avanti they can check with the RDG if they want to know for sure. Reservations are not mandatory for Avanti (or most British trains); also, once you're in GB, you can make reservations for any train at any ticket office.

As @rvdborgt  said UK is still in the Pass Validity area i could reserve many seats quite easy via Facebook or Twitter they may ask for a Photo of your pass :)

Thanks for the info, I was able to reserve seats through this link:

https://seatrez.com/seat-reservations-for-the-united-kingdom/

The general link https://seatrez.com/ that is on the eurail homepage seems to have been hacked for malicious purposes.