In reply to BruceM:
AIUI, if you have an acount (e.g. an investment ISA) ) with a provider (e.g. Barclays Stockbrokers or AJ Bell), you can hold a range of investments (e.g. Vanguard fund or Blackrock fund) in the ISA. The FSCS protection & limits would seem to relate to the total value of the ISA (not the individual holdings in the ISA).
Also, if you have shares/funds in an ISA, those shares/funds will (have to) be held in the broker's nominee. The level of protection provided by the broker's nominee is a murky world. The broker will proudly tell you your investments are "ring-fenced" if the broker goes bust, but there have been a few cases recently (e.g. Beaufort and SVS) where brokers have gone bust and clients have struggled to extract their holdings, and some have even nearly taken a haircut due to liquidators' fees being taken off the top (unless the FSCS stepped in and covered some of those losses). All my not be as it seems
I am not a lawyer nor an IFA, and you shouldn't rely on financial advice on an internet forum DYOR