The membership provision in the UKIP constitution states:
4.1 Membership is open to UK citizens and resident foreign nationals over the age of 16 years who share its aims and who are not members of any other political party or organisation membership of which the National Executive Committee ("NEC") has declared is incompatible with membership of the Party. Associate membership may be made available if thought appropriate to persons not qualified under the preceding sentences.
4.2 If a member of UKIP subsequently joins such a party or organisation which the NEC has declared to be incompatible with membership of the Party, or if the member is found to be a member of such a party or organisation then their membership of the Party will be automatically revoked.
4.3 If a member of UKIP is a member of an organisation or party which has been declared to be incompatible with the membership of UKIP by the NEC during the currency of their membership of the Party then that member will be given 28 days within which to leave the said incompatible organisation or party. If he or she fails to provide evidence of such withdrawal to the Party Secretary within 28 days his membership of the Party will be automatically revoked.
The above provisions state that someone who is not a member of a political party which the NEC has declared is incompatible with membership of UKIP could join UKIP.
The prohibition found at http://www.ukiplocal.co.uk/helpukip, which states:
Membership is not available to anyone who is or has previously been a member of
- the British National Party,
- National Front,
- British Freedom Party,
- British People's Party,
- English Defence League,
- Britain First or
- the UK First Party.
Any applications made from people who are or have been members of these organisations will be refused, and any subscriptions collected will be refunded.
By applying for membership you certify that you are not and have never been a member of either of these parties.
is, because it incorporates an additional condition not stated in the UKIP constitution which is also retrospective, ultra vires.