Further to an exchange on Twitter apropos of (didn't see this, only reported) instance in Downtown Abbey of an unmarried upper-class woman getting herself fixed up with contraception before having a fling. Well, not entirely improbable, but I really don't think she would have been sending her maid (even if she trusted her implicitly not to blackmail her further down the line) to procure the necessary.
The most reliable form of contraception for women being advocated by e.g. Marie Stopes, and indeed the rest of the UK birth control movement, was the female occlusive rubber cup, and while there were controversies between Stopes's pet high-dome cervical cap and the Dutch cap (diaphragm) that other clinics thought women found easier to fit, it still had to be fitted by a trained nurse or doctor and this was hardly something that a maid could undergo on her mistress's behalf. What the lady in question needed was one of the discreet Harley Street specialists unconstrained by the requirements of the various voluntary clinics to ascertain that all their clients were respectably married. There would, however, be practicality issues around fitting a woman who did not have former sexual experience.
There were other possibilities: W J Rendell's 'The Wife's Friend' quinine pessaries were widely advertised (though without ever explicitly stating what they were for and why they were the wife's friend) in the most reputable women's magazines. However, this is an under-researched (and possibly impossible to research) area of contraceptive practice - the oral histories studied by Kate Fisher in Birth Control, Sex, and Marriage in Britain 1918-1960 and by her and Simon Szreter in Sex before the Sexual Revolution are silent on the topic. However, there is evidence that this was a profitable line, and Rendells were incensed when the National Birth Control Association (later the Family Planning Association) found them not meeting the standards of efficacy and safety necessary for inclusion on their Approved List of contraceptives, pointing out that they were widely used by numerous (they claimed) entirely satisfied customers (documented in the FPA archives in the Wellcome Library). Quinine, however, is only weakly spermicidal, although the greasy medium used as a carrier did have some effect in slowing sperm motility. But anyhow, these would not have been a particularly reliable expedient.
The really sophisticated cosmopolitan woman might hear about and resort to one of the early forms of intrauterine devices that had been around since the early 1900s, principally the Grafenburg ring, named after its deviser. There was, allegedly, only one doctor (and to no-one who knows anything about these issues surprise, it was Norman Haire) in England who would fit them, and he charged the kind of rate that that commanded, to the extent that at least one woman (the bohemian writer Ethel Mannin, as stated in her correspondence with Douglas Goldring) found it slightly less expensive to travel to Germany and be fitted by Grafenburg himself. Mannin was a great advocate for this method even while conceding that the process of fitting it was the reverse of fun and might even require local anaesthesia.
I find it hard to suppose that the maid's errand would have been to purchase condoms to be discreetly handed over to the partner at an intimate moment. This does not seem a likely scenario for the period.