Cut the tap off from the top with a 115 mm angle grinder, as close to the
flange as possible. Then grind out the centre of the flange to separate the
threaded part (underneath) from the flange. Then you should be able to prise
the remains of the flange off without damaging the bath. Then you can get
penetrating oil on the interface between the nut and the bottom of the bath.
This will be "glued" to the bath with rust and / or sealant. If it is obviously
rust you could also try phosphoric or hydrochloric acid, if you know how to
handle them. Once it has soaked, use a hammer and drift from above on the
threaded portion. Cast iron baths are normally surprisingly strong, if you have
ever tried to break one up with a bolster or sledge in order to get it
downstairs!