This phrase is being used with regards to the valuation of the pound sterling as a currency. This excerpt talks about the potential negative consequences Britain leaving the EU will have on itself economically, including the effect it will have on the valuation of its currency. Often, when something negative happens to a nation's economy, the valuation of its currency will go down in value, but will usually stabilise and stop going down in value over time.
To say that the pound sterling will struggle to find a floor means that when the value of the currency presumably drops due to Brexit, it won't get to this period where it stabilises (the floor) and will continue to drop over time. The reason this metaphor is used is because the floor (the rate stabilising) is a state that you (as an economy) would like to reach, but are struggling to attain.
I would liken it to the concept of you falling into a dark hole. You want to hit the bottom as quickly as you can, because that means your fall is smaller and it's easier to get out. If you're falling and you can't find the bottom, that means that hitting the bottom is going to hurt you more, and it's going to be harder to get out once you do.