It depends. You should seek professional advice on this. You haven't posted enough information to make the situation easy to assess, but I've given the most usual scenarios below.
If you are not jointly on the mortgage with your girlfriend, and are not claiming any beneficial interest in her house - i.e. if your contribution is by way of rent, rather than direct mortgage payment - then you may be treated as a first time buyer. This would probably assume that she's staying in her house, and you're buying a house she's not going to live in (i.e. new purchase after break-up or complete change of relationship circumstances).
If, on the other hand, the two of you are planning to sell a house that's in her sole name in order to buy a house that'll be in your sole name purely to benefit from a stamp duty exemption, you actually won't be treated as first time buyers. Revenue rules are quite clear on this.