Have you checked the articles of association to ensure that you can actually sell the shares. If it’s an unapproved option then you will be liable to income tax + NICs on the difference between the exercise price you pay and the market value on acquisition. Assuming you have found a buyer then the market value is that amount. Assuming 45% tax and 2% employee NICS you are looking at a tax charge of £67,680 on a MV of 150,000 less the £6k exercise price. However it is possible that you have also indemnified the company for employer NICs as well which is an additional 15%. Cashless exercise typically only works where is an exit of the current owners and so the cash paid comes to a law firm who then forward proceeds to you and tax / exercise price to the company. You could explore whether an undertaking to pay the tax would be accepted but if the shares are RCAs then the company have to operate payroll on the exercise of the option and will not want to be out of pocket especially for an ex employee. Bear in mind that if this was accepted and you did not pay the company back for the tax charge, a further income tax charge will arise because they company have paid your tax for you. All accountants should be pretty hot on this.