There is no definitive approach to the allocation of equity between UK founders. Founders should discuss their respective contributions and commitment to the business and allocate equity on that basis. The standard approach is to have an equal split between founders, but that split is best if it is the result of a reasoned decision-making process about the relative value brought into the business, the future role expected, time incurred on management of the business, etc. It’s important that founders keep in mind the impact of future dilution, including the need to reserve some equity to incentivise early hires from the company’s option pool and any shares that may be issued on future funding rounds.
It is also common for founders to have reverse vesting imposed on their shares, so that if a founder leaves the company and is no longer adding value, their shares can be "clawed back" and potentially used to incentivise the remaining founders or a replacement. This is addressed by way of the conversion of all or a percentage of the founder’s shares to deferred shares (which have no economic or voting rights).
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