As @phildawson has explained excellently, OTC listed stocks are not permitted on ISA. If a stock is delisted from its regular exchange and it is moved to OTC markets, the open positions in ISA will be liquidated in the time span of 30 days, unless closed from your side. Positions in the Invest account will not be affected. For further reference:
@Mr.Trade Orders on Invest/ISA are routed to the most favourable exchange and are executed on the best price that satisfies both the buyer and seller at the moment of execution. Therefore, the execution of the order can occur via OTC so that the best price is obtained.