When you say bitcoin can drop far lower then gbp. What bitcoin is at near £3k to a pound. What you suggesting bitcoin 0.90 to £1 pound.
Sorry but this is not possible. Cant see this happening. Yes may drop but never this low.
I mean the percentage drop. Bitcoin can easily use 50% of its value while in case with GBP something more serious than brexit should happen to make pound lose its price so fast.
No one cares about the value of BTC. We just need to see how it behaves over time and GBP is much safer.