On Britannia in the Baltics and in summary thus far:
Kiel - no shuttles, docked in the centre, leave the port, head right and follow the blue line in the pavement. No shuttle bus needed and none provided.
Stockholm - shuttle provided and £5 per person each way. Select was free, Mrs RL and me were not charged and are on a Select fare. Journey time was 15 mins and drops off / picks up outside the Opera House. Plenty of buses and coaches, buses certainly accessible to all but could only take one wheelchair at a time.
Helsinki - shuttle provided and £4 per person each way. Select was free, Mrs RL and me were not charged and are on a Select fare. Journey time was 10 mins and drops off / picks up outside Makasiiniterminaali. This is a 2-3 min walk to the market and 10 mins from the main department store. Plenty of buses and coaches, buses certainly accessible to all but could only take one wheelchair at a time.
Cards scanned getting on bus in both directions. Always a bus waiting at port and in the towns / cities. Masks not mandated and no warned by most.
As ever if you are tall and getting on the bus (rather than the coach) then mind your head getting into the back seats and also watch for luggage racks above in any seat.