Also you are selling your 50% shareholding to an existing shareholder to make them a majority shareholder, therefore premiums may be applied in certain circumstances.
Or of course discounts because there is limited open market for people who will own only 1/2 the company and the headaches that go with that. Not to mention that your shareholders agreement or articles may have restrictions on who can can’t sell what without the other’s cooperation.
You probably do need an accountant at some point to make sure the tax value is handled correctly, but it’s probably easier to tell them your intended saw price (and any rationale) and let them agree it with the revenue rather than vice versa.
Since you are in the business it should be possible for you to work out what it is worth to you, and to the other party. I am sure all sorts of personal circumstances play a part in this – why do you want out? why does he want to own it all? what will you be doing (competing?) next? Will any staff follow you there eventually? what will you use the money for? Can he get his hands on the cash? How quickly do you want to exit? How would any customers / staff perceive your exit? What is the company’s debt position and forward order book like? Do either of you have any personal guarantees etc.? Does Brexit or other market changes present a risk to the business? do the two of you actually still get on? Etc. You need to be able to have that discussion with someone who really understands those issues and I suspect a corporate lawyer may be as well to advise as an accountant.
At the end of the day a starting point in the discussions would be: if he walked in tomorrow and said he wanted to leave what would you be prepared to pay him for his half to run it yourself, or in order to sell the whole business on 6/12 months later.
Depending on the situations it may be that some sort of “earn out” arrangements are actually better for both parties.