have BA let us down

You would probably get a more informed answer to the reason for the change on the Flyertalk forum. There is actually a similar question about the flight to Vancouver right now, although that person is not in World Traveler Plus.

https://www.flyertalk.com/forum/british-airways-executive-club/2170247-ba-changed-my-seats-heathrow-vancouver.html

The 777-200 is one of the workhorses of the BA fleet. We have had that plane swapped in for the 747 they used to fly to and from Seattle. I don’t know why they would switch planes for your Vancouver flight, but it may be related to ongoing problems with the A380’s. About a third of that fleet seems to be out of service due to maintenance problems, so flights have to be canceled. Perhaps they are reshuffling the planes to put A350s on the A380 routes in the future.

I’ll bet you can a better answer as to “why” on Flyertalk. But most of the comments on the thread above suggest that while the A350 is “quieter”, the seats on the 777 are more comfortable. Also the wider aisles on the 777 make for better meal service. At least one person on that Flyertalk thread says they go out of their way to avoid the A350.

How have they let you down? You still have your flight, and apparently without a time or seat category change. All they've done is change the type of aircraft. This is far from unusual and occurs with all airlines who use different makes and models of aircraft on their routes. Aircraft may be shuffled around for many diffrent reasons - maintenance schedule changes, various operational needs. BA doesn't have very many A350s, but has a boat load of 777s, so substituting one of those is no surprise.

Many of Delta's A350's have a very comfortable Premium Select section that I seek out over any other models. However, they also purchased used A350's that do not have a premium select section and I avoid those unless I want to afford first class. The A350 has the best climate control as humidity is supposed to be better maintained. In general I prefer them over Delta's older 767's which are really aging. It's a tossup for me between the A330 and 767 - both are aging and rather crowded with very small premium select sections. I also hate it when planes are swapped after I have made seat selections. That can lead to some really unexpected and annoying neighbors! One couple was split up and I got the chatty retired teacher next to me while her husband relaxed in peace and quiet in the row behind us. I got some "haha" looks from him and my partner as well.

When you book almost a year in advance, it is pretty common to have both schedule and equipment changes. The fact that they’ve switched to a smaller plane probably means that they don’t need as much capacity on your route. When is your flight? Around October as peak season travel wanes is when a lot of airlines start downsizing flights to Europe.

Personally, unless the seat size and legroom is less, I don’t care what kind of plane it is. Both of these planes have World Traveller Plus in a 2-4-2 arrangement and the seat size and pitch is identical.

The 777-200 looks to have fewer seats than the 350-1000, so it could be the demand did not justify the larger capacity plane.

https://www.aviatorjoe.net/go/compare/777-200ER/A350-1000/

Thanks for all the replies, great answers , think it was the disappointment of not using the a350 as last year the substituted a380 to a a330 fron American. Ah well still have top seats and same times to meet our connections to and from Manchester.

It looks like someone on FlyerTalk has a possible answer as to “why”:

Looking at Google flights, on some days of the week they've added a second flight to Vancouver, the BA87. So to avoid increasing capacity too much, they've swapped the A350 for something smaller on the BA85.