Think that is illegal in the UK now - all adverts must be at the same volume as the actual broadcast.
Well, it's hard to define loudness. In digital audio, there's a sampling frequency (the higher it is the higher frequencies you can reproduce. Keep in mind that lower frequencies have high harmonics that you can't really hear, but it makes a difference, when they're gon) and a bit depth which tells you, how many different loudness levels you have.
Now 44.1 kHz, 16 bit is fine for average music, it's almost not enough for classical music where you have some very quiet parts and some very loud ones. With compression (not the same thing as making an MP3), you can get those faint sounds louder while keeping already loud sounds at the same volume. You need to look at a small bit of audio, say half a second, else it sounds very unnatural, you compute the average volume of that bit and raise the volume if it's below the set threshold. The downside is that explosions and such don't make you jump out of your seat because they're not really louder than a conversation in a movie.
So broadcasters have to find a balance between compression and original volume to keep the volume high enough without killing the dynamic. Advertizers, on the other hand, will crank up the compression all the way so everything is as loud as possible, the colors are exaggerated and the whole screen is taken up (this is why the channel's logo has to go away during commercials: advertizers pay to get the whole picture). If you're a magazine and you print an ad and it's not exactly the colors your clients specified, they will pay you much less for it.
I imagine in the UK the ads are made more quiet to match the broadcast. Wish we had that law in Germany. It's one of the many, many reasons I don't watch regular TV anymore, but it's still annoying when I'm at friends' places and they keep the TV on for "background noise".