Can I hold multiple currencies on my account?

Yes. You can hold a balance in any currency that you choose Manual currency handling.

If you choose to deal with Manual currency holding there are a few points to consider:

With Manual holding turned on for a currency, purchases will no longer be charged in your base currency. As, such it is possible to run a negative balance in another currency. You can review your Cash Fund Overview by clicking on the cash funds value on your portfolio.
Because most of the fees that DEGIRO charges are levied in the base currency of your account (GBP for the UK platform). As such it is necessary to keep a sum of money in GBP on your account to cover these fees. Otherwise it may be possible for an order to be rejected in the case that the commission fee cannot be levied.
If you switch from Manual to AutoFX then any foreign currency will be converted back to the base currency of your account.
When making a withdrawal, funds must be converted back into your base currency.