The Pen Shop, which I believe is located in Debenhams (Meadowhall). They will have a good range, and so you can have a good play and see which you think she might like.
Everyone's standards are different for what they consider to be a daily writer – my boss uses anything from a Lamy Safari (£15), through Parker Duofold (£250) to special edition Mont Blancs (£300+). that's nothing compared with some Conway Stewarts….
My "best" pen is a ltd edition Dunhill Sidecar with palladium cap, but that is definitely not a pen I would take out of the house. It writes beautifully.
My three daily writers are a Cross Century Classic (a graduation gift several years ago, and a fantastic writer), a Japanese-only Spalding Brothers wooden bodied pen (cheap, but a nice line for work), and a Lamy Vista (£15, gets knocked around bags and places).
Assuming you're not really planning on spending more that £100 (I may well be wrong), then brands to consider would include Parker (have a look at the Sonnet, a classic), Cross, Waterman, Sheaffer, Lamy, Pelikan (really nice writers, though I'm less keen on the looks).
If you're spending more, then continue with Parker, Lamy and Pelikan. Also consider Yard-o-Led (made in Nottingham), Caran d'Ache (some very "fine" looking pens, and all write well), Mont Blanc (I'm not a fan) and, well, the list goes on.
Can you tell I like writing with a fountain pen? 😀