Here are a couple where an actress with a speaking role still (as far as I know) remains unidentified.
Series 1, Episode 1, (Jan 2007)
The identity of the actress playing the woman in the house opposite to Tony who parades around naked remains a mystery. She has some dialogue and her husband calls her Miriam - however she is not mentioned in the end credits.
A bit confusingly there is another completely different off-screen "Miriam" mentioned in the same episode (a schoolgirl friend or sister of one of the guest characters) which seems an odd scripting conflict since there was no story reason for it. It's possible perhaps that the name wasn't scripted and the exhibitionist woman was called "Miriam" because that was her real name used for convenience - but that's only a wild guess.
She seemed to be a capable actress (rather than a model) but I've never noticed her in anything else. The actor playing her husband was John Arthur and he was credited as "Middle Aged Teacher" although he was in another scene as well with a larger role. Neither she nor he appeared in any subsequent episodes.
Dialogue that mentions the other Miriam:
ABIGAIL: (to Tony) Are you sill coming to my party tonight? It would be safe if you did. My parents are in Cloisters.
TONY: Sounds great.
ABIGAIL: After 8.30 because Miriam has to get back from her cello recital.
HUSBAND: For God’s sakes, what are you doing, Miriam? Why do you have to get up so early?
MIRIAM: Just getting ready.
HUSBAND: Anybody would think you wanted the neighbours to see you. Do you hear me, woman?
MIRIAM: Yes, darling.
See Blog Post: Now Identified: Uncredited "Miriam" Character from Skins Episode 1
Living TV Trailer
(The earliest reference I have to this was in Nov 2000, so it may be from then or a bit earlier)
A young woman dashes from her ground floor flat wearing just her briefs to collect her morning post from the communal front door mat. But her flat door slams shut behind her and she is left stranded in the communal hallway severely underdressed - then to add to her woes there is a ring on the front door bell and she hastily decides to hide in the understairs cupboard until it is all clear whilst a male tenant from upstairs comes down to answer the door - but the caller turns out to be a meter man and her hiding place is where the meter is located and when they open the cupboard door they discover her and she tries to act nonchalantly about her predicament.
This was a specially made trailer meant to encourage viewers to watch the Living TV satellite channel. It was not (as far as I can tell) a trailer for a specific programme.
UPDATE: She has been identified as Helen Goldwyn (see Comments)