It depends on when exactly the party is supposed to have taken place, but if you take the Regulations as they applied on 20 December, for example, the relevant Tier 4 restrictions (which applied in London at the time) were:What was the exact wording of the rules?
Paragraphs 1(1) and (2) of part 1 of Schedule 3A:
(1) No person who lives in the Tier 4 area may leave or be outside of the place where they are living without reasonable excuse.
(2) For the purposes of sub-paragraph (1)—
(a) the circumstances in which a person has a reasonable excuse include where one of the exceptions set out in paragraph 2 applies;
Paragraphs 2(1) and (5)(a):
(1) These are the exceptions referred to in paragraph 1.
(5) Exception 2 is that it is reasonably necessary for P to leave or be outside P’s home—
(a) for the purposes of work or to provide voluntary or charitable services, where it is not reasonably possible for P to work, or to provide those services, from home;
And also the gathering restrictions, which similarly stated, in paragraph 3(1) and (3) of part 2:
(1) No person may participate in a gathering in the Tier 4 area which—
(a) consists of two or more people, and(b) takes place in a private dwelling or in any indoor space.(3) Sub-paragraphs (1) and (2) do not apply if any of the exceptions set out in paragraphs 6 or 7 apply.
Paragraphs 6(1) and 6(5):
(1) These are the exceptions referred to in paragraphs 3, 4 and 5(5)(c).
(4) Exception 3 is that the gathering is reasonably necessary—
(a) for work purposes or for the provision of voluntary or charitable services;
So in essence, the key test is whether the gathering (or leaving or remaining away from home) was reasonably necessary for work purposes and whether it was reasonably possible for the people involved to work from home.
A gathering which purely served a social purpose would be unlikely to fall into those categories, and could reasonably have been done remotely in any case.

