Yes, that's probably the best bet. You can tell the taxi driver to drop you off at the Regal Riverside Hotel (Stop #12 in the above link) or the Che Kung Miu (Temple) - Stop #16. Big landmarks close to the tunnel.

I am not sure, but I'd think there may be red public light buses (i.e. death machines) that goes on Tai Po Road (on the hill-side of the CU Campus) direct to Hong Kong Island. If you're brave enough... I am not.