Is this data extrapolated from the corresponding number list? If so, how is anonymity maintained?
Through convention. It is in fact possible to link an individual to their vote, it is simply against the law to do so except under authorisation from the Courts or Parliament. This is governed by the Representation of the People Act 1983 , which contains amongst other things a prohibition on electoral officers attempting to make the link between ballot paper and the tear-off slip in the way you describe.
Returning officers, candidates and other election officials are barred in Section 66:
shall maintain and aid in maintaining the secrecy of voting and shall not, except for some purpose authorised by law, communicate to any person before the poll is closed any information as to—
(i) the name of any elector or proxy for an elector who has or has not applied for a ballot paper or voted at a polling station;
(ii) the number on the register of electors of any elector who, or whose proxy, has or has not applied for a ballot paper or voted at a polling station; or
(iii) the official mark.
People attending the count are barred in Section 66 (2):
Every person attending at the counting of the votes shall maintain and aid in maintaining the secrecy of voting and shall not—
(a) ascertain or attempt to ascertain at the counting of the votes the number other unique identifying mark on the back of any ballot paper;
And everyone else in Section 66 (3):
No person shall—
(a) interfere with or attempt to interfere with a voter when recording his vote;
(b) otherwise obtain or attempt to obtain in a polling station information as to the candidate for whom a voter in that station is about to vote or has voted;
(c) communicate at any time to any person any information obtained in a polling station as to the candidate for whom a voter in that station is about to vote or has voted, or as to the number or other unique identifying mark on the back of the ballot paper given to a voter at that station;
Regarding postal votes...
Every person attending the proceedings in connection with the issue or the receipt of ballot papers for persons voting by post shall maintain and aid in maintaining the secrecy of the voting and shall not—
(a) Except for some purpose authorised by law, communicate, before the poll is closed, to any person any information obtained at those proceedings as to the official mark; or
(b) except for some purpose authorised by law, communicate to any person at any time any information obtained at those proceedings as to the number or other unique identifying mark on the back of the ballot paper sent to any person; or
(c) except for some purpose authorised by law, attempt to ascertain at the proceedings in connection with the receipt of ballot papers the number or other unique identifying mark on the back of any ballot paper; or
(d) attempt to ascertain at the proceedings in connection with the receipt of the ballot papers the candidate for whom any vote is given in any particular ballot paper or communicate any information with respect thereto obtained at those proceedings.
Emphasis mine. So yeah, it's certainly technically possible, and a court might possible order it, but otherwise it's comprehensively illegal.
