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At risk of stating the obvious, were you comparing similar shot weights? I have always thought that many British loads were a bit anemic compared to US loads.

For comparison:
1oz = 28gm
1 1/16oz = 30gm
1 1/8oz = 32gr
1 1/4oz = 36gm

Good quality UK and Euro 1oz / 28gm trap loads can run anywhere between 1200 & 1400fps.

32gm game load generally around 1050-1100 fps.

You are right. Some of those claimed velocities are high. It is quite a few years since I shot clays with any amount of dedication.

I have been ignoring the technical side of shotgunning for at least ten years.

Maybe propellants have improved?

I have no opinion on whether the published velocities are correct or not BUT it is entirely possible to achieve faster velocities in a 12 ga with reloads. I recall reading an article in Handloader 25 years ago or so that loaded some 1 1/8 12 ga loads up to about 1550 FPS with safe pressures. I couldn't wait to reload some and did as soon as I got home. I actually read the article at a trap shoot in Houston. I sure am not going to quote grains of powder and I am not even certain the powder is correct at this remove, but I think it was 4759.

At any rate, I loaded some, and when I touched them off on the skeet range, a couple of people a hundred yards away came running up worried that a gun had blown up. It hadn't, but the recoil was pretty fierce and the noise was impressive. I shot the box of them up, readily sharing them with adventurous friends and decided I didn't really need that much "umph" on either end of the gun.

Basically, using lead and light shot, initial velocity is not as big a factor as one might believe. I've killed a lot of pheasants with 1 3/8 ounce starting at about 1100 FPS....super pleasant to shoot and smokes them. I like 1 3/8 as a good all around wt for almost anything that flies.

A lot of steel shot loads in the US are pushing 1500 fps and faster. When shooting lead one dosen't need all the extra speed and recoil.

MY favorite grouse load is 1150fps and I've kill hundreds with it. My pheasant load is about 1300 in 12 ga. But then I v'e killed hundreds of them with a 1 and 1/8th oz out of a 20 ga at 1150.

The faster steel loads do kill much better then the lower steel loads.