GBP has long been a safe haven currency, especially for those heavily into EUR. The UK had by far the strongest economy that was also coupled with its own local currency in Europe, and this was growing at the fastest rate in the G7. The political uncertainty over the terms of Brexit now cancel that advantage for the GBP.
To some extent, internal political uncertainty is also increased, with a governing party that is split and not obviously sure of how to proceed, plus more widely separated leftist and rightist opposition parties than for many years in UK history.
On top of this, we are going to be moving from a free trade co-operative situation to a competitive trade situation, with the possibility of tariffs and restricted movement of labour which UK industry currently benefits from. This suggests slower economic growth and points attention back to our national debt level. Even for risk-accepting investments, the UK must now be seen as less attractive than it was for the foreseeable future.