From the FAQ:
For newly added cards, most of our users will have a top-up limit between £1000-£3000 (or currency equivalent) per 24 hours. Some users may find they have a lower daily limit to begin with, however, your daily top-up limits will increase over time. We suggest:
Try and top-up your desired amount. If the amount is over your top-up allowance, you will receive a pop up message notifying you of the daily limit for that card and how much you can top-up today. If you wish to top-up more than your daily limit, you can use another debit or credit card, alternatively top-up via bank transfer. Unfortunately for security reasons we cannot provide the specific details around how your top-up limits are calculated.
So there you go, be patient and your limit will increase in time
As you seem to be in the UK, bank transfer are usually pretty quick, you just give that a go as well