Whatever asset class you choose your investment will only be worth what people are willing and able to pay for it. Some things are a safer bet than others.
If you want an idea of how currency will do long term have a look at the trade surplus/deficit, dependence on imports and sources of revenue – edit: and inflation. For the UK:
1. An ever increasing trade deficit.
2. Increasing dependence on imports.
3. Declining revenues from North Sea oil and gas, banking and finance, pharma and what's left of manufacturing. Those are the things that have kept Britain afloat for the last 20 years.
I have no pound denominated investments whatsoever. To lose my bet the pound will have to get back over €1.50 which I doubt.