The Everything list of modern British, that is United Kingdom, political parties
The Registration of Political Parties Act 1998 established The Register of Political Parties. (Unsurprisingly). Registration is not a requirement and remains entirely voluntary, but it confers certain advantages, notably allowing a party some degree of protection over its name (and thereby preventing any reoccurrences of the Literal Democracy affair of 1994).
The party names on this list are the formal registered names for each party and therefore often include any alternative name under which the party may wish to campaign.
This is, generally speaking the Welsh equivalent. Or in the case of the two Welsh nationalist parties and the Cornish nationalists, who have a different set of priorities, the English equivalent. (Although the Cornish party, Mebyon Kernow, helpfully includes the English translation since no one much actually speaks Cornish these days.) Note that no one really bothers with the Scots Gaelic equivalent. There are presumably no votes in Scots Gaelic.
As the very act of registration itself can appear to confer some type of legitimacy, it appears as if every remotely organised group of people have staked their claim to the attention of the British electorate. The list therefore includes a number of residents associations and other local groups who are only active in a limited locality, every local party organisation that is currently at loggerheads with party headquarters, as well as every kind of single issue campaign and encompasses almost every conceivable nuance of political thought. (And it should be noted that many consist of nothing more than two men, a dog, a photocopier and a phone line in an office above a newsagents somewhere in East Grinstead.)
It remains however an interesting snapshot of the British political landscape. In the interests of clarity, those political parties that appear to be of any minor significance whatsoever have been highlighted in bold. (Or in my opinion anyway. You can always \msg if you disagree.)
The Everything unexpurgated index of Registered British Political Parties
Sourced from the official UK government list at http://www.party-register.gov.uk/register.html
A note to any aspriring linguistic pedants. I know that for example, it's the Stamford Ward Residents' Association but the Upminster and Cranham Residents Association. Can't help that, those are the names that were registered. Presumably the residents of Stamford have a different take on English grammar from those of Upminster and Cranham.