I've only been at RAH a couple of times. Most recently was a couple of years ago and I think the acoustics have been improved over an earlier visit. However, the basic huge size and circular shape of the RAH are really hard to deal with. It's also a good venue for circus acts and ice skating shows. We've been going to the relatively new Cadogan Hall, near Sloane Square, the past couple of years where the RPO seems to have taken either primary or at least a secondary residence. It is smaller, at 900 seats, than Barbican or RFH, and seems to suit their forces pretty well.
Wigmore is by far our favorite London venue - intimate, with great acoustics for solo or chamber music or even medium small ensembles. It is only about 6 blocks from our London timeshare, and usually has 9 concerts a week - so there is plenty to see and hear. They even have a nice little restaurant downstairs for a pre concert dinner. My only complaint is that at 6'2", my legs are not designed for the row spacing - like economy on typical US domestic airlines. Each year we go to Wigmore, the audience has seemed to aged a year - more walkers and wheel chairs. We even had a concert stopped this past year for announcements for the person with the hearing aid that was making a loud, high pitched noise, to please turn it off. I'm afraid the plea fell upon deaf ears!
Larry