I would agree that Universal's park in the UK is more of a day trip than a multi-day resort, which is what Disneyland Paris offers. I guess some UK guests, though, might decide to save their time and money and just take the family to Universal rather than travel across to Paris.
Maybe it's just me, but when I look at Universal's plans for their UK park I can't help that think Disneyland Paris' second park will be pretty competitive with it in terms of offerings and theming by the time it opens. Universal will be cheaper and more convenient for UK visitors which does make a big difference. However, based on the plans and artwork that have been released, I get the impression Disney's Adventure World (

) will actually feel like a more well-rounded park with arguably more headline attractions than Universal UK. This is where I don't see many people travelling specifically to the UK to visit the park: it seems far more pitched to the domestic market and as a day trip for families already visiting London.