Be warned - there have been a large number of services between London and Norwich which have no catering recently, and have reduced First Class without tables and charging sockets. I have had two such trips when I've been using a First Class Advance ticket, and on both occasions my request of the station staff to wait for the following service, which was confirmed to have the onboard catering and table seats, was declined. Complaints were made on both occasions, and I was simply reminded that this is company policy at Greater Anglia.
If this frequent substitution of rolling stock is continuing now and into February I don't know, but perhaps others can advise.