None of the three places you mention have hills to the west of them. If they did, they would be in the lee of the high ground and thus drier - the prevailing winds in the U.K. are westerly, most weather systems come in from the Atlantic.
The broad pattern in the U.K. is that the west is wetter than the east. The wettest places are high ground in the west.
Having grown up in the Pacific Northwest US, where grey skies are most comfortable to me (and my eyes have always been light-sensitive) I completely sympathize with that outlook.
I talk all the time about how I prefer filtered sunshine, and that’s not sarcasm.
Oh FFS - yes they do. The people of Huddersfield will be very amused at your bold statement. You don’t need to explain to me about prevailing weather systems in the UK and the fact that it’s drier in the east. What is fairly clear when you look at relief and rainfall maps of the UK is that to get the benefit of being in the lee of hills you have to be at some distance from them. If you’re quite close to the eastern edge of the hills you will still get dumped on. The west side of Huddersfield gets significantly more rain than the east side.
Admittedly the hills near Cardiff are more NW than west but it looks to me like Cardiff still gets a soaking because of them. If there is some other explanation for why Cardiff’s annual rainfall is so much higher than that of nearby Bristol then that’s fine, I’ll be interested to hear it.
Galway - Galloway is in Scotland.
It’s likely true that Seattle gets more rain by inches than most places in the UK. But I think it’s fair to say that most of the UK gets more rain by time than Seattle. British rain is nearly always drizzle or at least light, except in the summer. American rain seems to come primarily in thunderstorm form.
Not in Seattle. Thunderstorms are relatively rare in the Pacific Northwest. And Seattle gets lots of drizzly days during its wet season.
Truth. Thunderstorms are rare enough here that they are like special events.
We don’t get a lot of really harsh rain. It’s usually little sprinkles.
You’re describing Seattle rain there. Actually, you’re describing what I see outside my window as I write this! 
That depends on location and time of year - I’m in NYC where it is not uncommon to have thunderstorms between May and late September/early October. And those thunderstorms may move quickly and last only a couple of hours ( or less) - but when it rains between mid-October and April, it’s usually steady rain for one or more days.
Yeah, in Phoenix metro we get 2/3s of our rainfall, such as it is, from November through March where it rains everywhere, all day, for a quarter-inch or a little more in 24-hours.
The other third is during the monsoon season from June through September where you get an inch dumped on you in two hours – if you happen to be in the mile-wide swath that passes through.
The UK and Ireland are often in line with the high altitude jetstream air current. It tends to push the remnants of Atlantic weather systems in a procession of two lows pressure systems for every high. This makes the weather highly variable with weather fronts coming thick and fast. Sometimes the jetstream swings south and we get a blast of cold northern air or if it swings north and it becomes quite balmy for a few precious days in summer. The gulf stream ocean current brings a lot of heat and keeps northern Europe ice free despite the high latitude. Though the rain is seldom a dramatic deluge, it is frequent and annoyingly unpredictable. For this maritime nation keen on gardening and outdoor activities the weather prospects are a matter of relentless speculation about what to wear. A fine summer day is a precious thing and the daylight hours are long. Winters are cold and grey and short of daylight. Seldom with the kind of Christmas snowscape of a Dickens novel. When it does snow the kids go nuts with excitement.
The UK is not usually a place for climatic challenges and grand meterological dramas. It is temperate and green. Like most countries in Northern Europe, we know to head south for a holiday in the sun. In winter we are lucky to get a couple of weeks of light snow, so it is better to head for the Alpine mountains for ski resorts.
Direct sunlight, unobscured by cloud cover? UK and Northern Europe gets far less that the US. Nice map here.
According to the Wiki articles, London typically gets about 23 inches annually, which is the same as San Francisco, but the latter figure really is from before the megadrought. I don’t think San Francisco got anywhere near that much over the past year.