If your father left a will, naming you as executor, then you can obtain 'grant of probate' fairly easily (in most cases) by filling in a few forms and swearing an oath (or affirming) at the local office of the Probate Service:
http://www.hmcourts-service.gov.uk/cms/wills.h tm
(Of all the 'official' bodies I've ever dealt with, the Probate Service comes out way ahead of the others, in terms of efficiency and helpfulness).
If your father died intestate then you can't obtain 'grant of probate' (which only applies to wills) and the best advice I can give you is to consult a solicitor.
Chris