I will not buy this pair if "Remain" wins because the market mostly priced in for a "Remain" win. It could be very well of a "Buy Rumour Sell News" case. The market may move up initially but then come down sharply with big players closing their position.
Alternatively if UK votes for "Leave", it will surprise the market hugely and there will be a huge sell off for the GBP Pairs. GBPJPY will be an ideal pair for a sell because of GBP weakness and JPY strength. This pair can move 1000 pips in one day if Brexit happens.
I hope liquidity will be available and not a repeat case of EURCHF drama.
Trade with care. Margin and spread will be higher.