ECF membership is not required for one day team events!

[Neill Cooper]

Well, unless you play 8 (or more) rapidplay games, at which point they need to be ECF Bronze members.
[This is obviously in contrast to one day congresses with 4 or more rounds where they would need to be ECF Gold members.]

https://www.englishchess.org.uk/members ... 024-25-v1/ [abbreviated] states:
2 – Leagues and other team events (including jamborees) which are not FIDE-rated
Players are required to be ECF members at Bronze level or higher. Juniors are eligible for the junior pay to play fee rate if they are under 18 years of age on the day before the league starts. Players who are not ECF members at Bronze level or higher do not pay any game fee for their first three* games in the relevant league’s competitions – but once they play a fourth game they become liable for a game fee of £20.00 if an adult or £6.00 if a junior.

Please note that the game fee of £20.00 or £6.00 applies once four* or more games have been played. It applies to all games played within events organised by a particular league, in whatever division, in whatever league format, or in a league’s team knock-out or jamboree events if any.

*Rapidplay games – references in this section to numbers of games played assume standardplay games. Rapidplay games contribute to these totals as half a game.