I’m a non-taxpayer unlikely to come close to triggering any annual tax threshold (divis, cgt, IT etc).
In a blue sky/Tesla world I’ve used ISA in case cumulatively a withdrawal in the medium to long term future from a taxable account was a taxable event (and simplify potential annual tax reporting for non-taxpayers 
- DYOR).
As a non-earner I minimise costs by using Trading212 ISA (free ISA, instant deposits, broad universe, long term profitable, platform diversity, FSCS cover for platform failure).
I use FT taxable account for stocks not available on T212; for instance I have just swapped a couple of grand over to FT for some EXCELLENT new tech etfs (market top, not investment advice! 
).
I love the ethos and community on FT and would happily swap more over as they beat the competition (Revolut, Stake, eToro, Robinhood (UK RIP) et al).
My Sipp is with Vanguard which is strictly vanilla buy/hold beta, up for grabs to a cheaper provider providing sufficient fscs diversification 
.