It is easy to switch between accountants, you get the new accountant to send the old accountant a letter requesting your documents and making sure there are no ethical reasons/objections why they shouldn't accept the appointment, and that's about it.
The settlor doesn't have too much to do with the trust, basically they contribute an asset (the settled sum), sign the deeds and can not benefit from the trust in the future so just make sure you retain all copies of the trust if you leave.
The cost can vary between accountants, I would expect for an individual and a trust with one IP you would need to ask prospective accountants.