The full text of paragraph 4 of the Regulations is here;
Participation in gatherings outdoors
4.—(1) No person may participate in a gathering in the Tier 4 area which—
(a)takes place outdoors in a place which satisfies the conditions in sub-paragraph (4) and consists of more than two people; or
(b)takes place in any other outdoor place and consists of two or more people.
(2) No person living in the Tier 4 area may participate in a gathering outside that area which—
(a)takes place outdoors in a place which satisfies the conditions in sub-paragraph (4) and consists of more than two people; or
(b)takes place in any other outdoor place and consists of two or more people.
(3) Sub-paragraphs (1) and (2) do not apply if any of the exceptions set out in paragraph 6, 7 or 8 applies.
(4) A place satisfies the condition in this sub-paragraph if it is a public outdoor place other than a fairground or funfair and—
(a)no payment is required by any member of the public to access that place, or
(b)the place falls within one of the following categories—
F2(i). . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
(ii)botanical gardens;
(iii)gardens or grounds of a castle, stately home, historic house or other heritage site.]
It doesn't apply in this case because of the requirement in paragraph 4(1)(a) that there are more than 2 people which I've added in bold above.