Hi Ally,
Welcome to DISUK!
It's great that you are thinking of visiting London and Paris for a vacation I am sure you will love them both, especially
Disneyland Paris which is, although much smaller and very different, in some ways better that WDW.
When you visit London at Easter it is hard to say what the weather will be like. Don't expect nice hot temperature though! We've had some Easters when it is fairly nice and sunny and some where it has been cold and rainy, you'll just have to wait and see.
Disneyland Paris does get quite busy during Easter but as the crowds will now be spread out over the 2 Disneyland Paris theme parks it shouldn't be too bad. Also, I'm not 100% sure, but I think our 2-3 week Easter break starts about 2/3/4 April, so DLP will have the 'Easter crowds' from the UK too, Im not sure about the French school dates though.
Have you thought about where to stay when you visit DLParis? The Disneyland Hotel is truly magical and very highly recommended by myself and DLParis deluxe hotels are MUCH cheaper than WDW deluxe resorts. A great website to visit is
www.dlp-guidebook.de and the official Disneyland Paris site:
www.disneylandparis.com
London has lots for you to do and nearby are lots of theme parks like
Legoland for instance, or there is lots of historic sites and great shopping within the city itself. If you want to know more information about what to do in London im sure lots of UK dis people can help!!
HTH
Richard