We did Oceania's 10-day "Celtic Kingdoms" back in August 2019. Southampton to Southampton, with stops in England, Scotland, and Republic of Ireland.
We're glad we did it, but the weather was c-c-c-cold. There's not a single, outdoor picture where we weren't bundled up. Also, the rain was frequent and some tours were relegated to saying, "If you could see out of the windows, you'd be looking at..." Our stop in Glasgow was bypassed because the weather was so bad that we couldn't safely enter the port.
I don't know if our experience was typical, but - were I to do it again - I would bring warmer clothing and more closely consult a weather site that contains historic data.