The London Marathon comes under the governance of WA, not UKA. Transwomen who have been through male puberty are banned by both bodies from competing in the female category.
The problem is far worse in cycling where national and world governing body policies diverge. So transwomen cannot compete in the female category in the British National Championships, but can compete in the Womens Tour, as while this is run by BC, it is done so effectively under "contract" for the UCI, and follows current UCI rules.
BC has taken a leap here and the other national federations in the 'big' cycling nations (France, Belgium, Netherlands, Italy, Spain) don't really have a history of independence - they just go along with what the UCI does. So it basically comes down to a battle in the UCI between BC and USA Cycling (who are completely captured). The UCI will likely follow the lead of BC (and other sports), and the other federations will all change their rules for domestic competition to follow the UCI (except for USA).