Might depend on who you've booked your advance ticket with.
If through Greater Anglia themselves, if you've an advance, and you decide not to travel, you should be able to exchange your ticket for the same origin/destination for another date/time. Might have to pay any fare difference if the new ticket is more expensive.
You are expected to do this before 6.00 p.m. tomorrow (Thursday).
Or if you don't yet have a new date/time in mind, it should currently still be possible to get issued with a rail travel voucher for future travel.
More info here...
https://www.greateranglia.co.uk/book-confidence
Alternatively, if Greater Anglia do actually issue a "Do not travel" warning between now and Friday (which they may or may not) and/or the booked train doesn't operate for whatever reason, you should be eligible to a full refund.
Anyone concur or opine otherwise?