The climate of the United Kingdom is notoriously variable and changeable from day to day. Weather is generally cool to mild with frequent cloud cover and rain, but occasional settled spells of weather occur in all seasons. As a general rule, the western side of the United Kingdom is cloudier, wetter, and milder in winter, with cooler summers than the eastern side. The eastern side of the United Kingdom is drier the year round, with a tendency for summer rain to be heavier than that of winter. You will rarely experience severe or unpleasant weather for long; however, you should be prepared for rapid changes of weather in all seasons. There is good reason for weather being a major talking point in the United Kingdom! ...
Try to insure that most of your garments are light-weight. They make denims that are made of a lighter-weight fabric. Same with slacks and tops. Maybe some winter silk tops in colors which are very light-weight even when you chose the medium-weight which I did. Perhaps a vest or sweater in fleece. I wore inter silks in Dec & Jan in MI, so they should help in the UK with it's dampness. It's wet there even if it isn't raining. The 20 days I spent in south Wales & Cornwall in August 2008 were very cold and Cornwall is the garden spot of England! My first trip I wore one pair of shoes (black "nurses" shoes) all three weeks. My second trip I wore them, took my sandals, and some comfortable heels. But I had trouble lifting the suitcase on and off the train. Fortunately my fellow passengers were very helpful. The beauty of the tour is there's someone else to do the heavy lifting most of the time. But there will be times you have to handle it and there will be two of you, sounds like. Have great fun. Ireland is one country I haven't visited yet, but it's on my list. Maybe next time.
BTW - I have a friend who's been all over the world - she says Sep or Oct is better because all the tourists have gone by then. August is vacation month in Europe.
I took the following from Tauck's "Before You Go," which is available on the tour itinerary page, found here.
The average temperatures when you're there are usually in around the low 60's / high 50's in Fahrenheit, or between 18-13 degrees Celsius.
I hope this helps!