Hi,

I haven't seen any mileage ticket with Island air and Hawaiian airlines together.Though, stopover is allowed, however united award structure and rules are different with the partners. As it's an award travel, no way HA and WP could be combined with UA under one ticket.So, it seems that it's a different award level and hence, charges would be there for bags intra Hawaii, if any.

Regarding comparing, BA with UA is similar to compare Apple with Oranges. Both airlines have it's own pros and cons. If i talk about hidden charges, BA have the most common hidden charge - Fuel surcharge which is often more than the base price

Thanks and Regards
taran_2005