I want to change the nameservers in 123-reg so that the website points to my webserver, but the email is being hosted by someone else.
Should I leave the @ and Mail records pointing to the present email hosting provider and just update the nameservers? Or should I add my nameservers to the records also?
Change @ and www to point your webserver(or only www, if don't use only domain name).
Do not change MX(mail) record, If you do not want to change ISP mail.