I wouldn’t regard Interactive Brokers as “middle of the road”. This US multinational has got $7.1B excess regulatory capital and you can trade in more than 150 markets (e.g. India and Hong Kong). According to
It is the largest electronic brokerage firm in the US by number of daily average revenue trades.
The UK arm provides an ISA. Be aware though for the normal investment account they do share lending however You can sell your shares at any time without restriction and can terminate your participation at any time for any reason. It provides you with interest if it does lend your shares. Pricing