If you use your ISP to provide your email service, you can access your mail through that ISP's website. (To the best of my knowledge
all UK ISPs offer web-based mail access, as well as allowing you to receive your mail via a dedicated client, such as Outlook Express or Windows Mail).
Just go to your ISP's home page and look for a link to email. You'll then be asked to enter your username and password. Doing so will take you to a web page from where you can send and receive mail. (When you've finished, remember to click the 'log out' button, so that no other user of the computer can access your account).
If you want to receive your mail, on another computer, using Outlook Express (or similar) you'll have to add your account details to that program. To do so, you'll need to know (as well as your username and password), the POP3 and SMTP server addresses. The relevant information, for most UK ISPs, can be found here:
http://www.kitz.co.uk/isp/dns.htm
and/or here:
http://www.idl.uk.com/email/isp_settings.htm
(If the information is not there, it should be available on your ISPs support pages).
When you've got the information, open Outlook Express (or Windows Mail, etc), go to Tools>Accounts>New>Mail and follow the instructions to add your account details.
Chris