A disposal, whether of the whole or part of the dwelling-house, is a relevant disposal for the purposes of this Part if it is—
a further conveyance of the freehold or an assignment of the lease, or
the grant of a lease (other than a mortgage term) for a term of more than 21 years otherwise than at a rack rent.
For the purposes of subsection (1)(b) it shall be assumed—
that any option to renew or extend a lease or sub-lease, whether or not forming part of a series of options, is exercised, and
that any option to terminate a lease or sub-lease is not exercised.