Civil partners are treated in the same way as married couples for tax purposes. These changes were dealt with in the Finance Act following the Budget of 16th March 2005. The HM Revenue and Customs department can answer specific queries about taxation law: www.hmrc.gov.uk.
If you form a civil partnership or a marriage you are treated jointly for income-related benefits - people should be aware that this also applies to same-sex couples who are living together, but who have not registered their partnership (as with cohabiting mixed-sex couples).
The HMRC also have produced a booklet for lesbian, gay, bisexual and transgender people on taxes and benefits issues, which you can download here.