You own the copyright. What you most likely want to do is retain the copyright, but assign rights to the clients to use those images. Unless you expressly sign over copyright to them, you will retain the copyright.
You can't resize an image into a JPG and give them copyright on that file. The copyright gives the copyright holder (you) the right to make copies, resize that image to different resolutions, alter the image to suit you or your clients, and to maintain control over the use of the image. It's not something someone has on a particular copy of an image.
You can grant them the right to use that specific file for a specific purpose (and say expressly forbid them from using a higher resolution version unless they enter into another agreement to use that version).
A contract is an agreement between two consenting parties. That could be handshake, oral agreement, it could be written and signed in person, it could be sent back and forth through the mail, or it could be emailed. It's the content that counts, and the fact that both parties have agreed to it. If your email simple says they agree to pay, but doesn't explicitly state WHAT the deliverables are and so on, then it's probably not a very useful contract. If the client says they believed they would own the copyright, or be able to use the images for some purpose, and you disagree, if you didn't cover all that in your email, then you don't have a sufficient contract IMO. Agreeing to pay is a very small part of that agreement.
I would contact a professional organization for photographers in the UK, such as the AOP. They have sample contracts and license agreements, model releases and so forth here: http://www.the-aop.org/information/downloads/legal-business-forms
You'll want a basic contract such as their Agent And Photographer Agreement (PDF) and a licensing agreement, such as their Licence To Use (PDF). Also note that they have separate terms and conditions documents for England, Wales, Scotland and Ireland.
There is a good overview of the terminology around usage rights here: Ad Agency Guide To Photography Usage Terms