Sysygy

I'd like to trade an equivalent to SPY (and a few other ETFs) in my ISA. If I l lived in the USA I'd trade SPY as it is massively liquid, tight spreads, no FX fees, good correlation to the S&P 500. If I get a dealing ticket up it denominates the trade in £, so I assume that a 0.5% FX transaction cost will be applied to my trade. Now from my research thus far I've found a lot of SPY equivalents but which one would offer me the lowest dealing charges (spread, dealing commission & exchange rate charge)? Also I'm not sure what effect Brexit has had on UCITS and MiFid, are these now defunct as far as the UK is concerned? Any help greatly appreciated.