I’ve been involved in lots of valuations. It would be incredibly rare to use a single method to value an unquoted share. For a majority interest in a trading company, dividend yield would be completely inappropriate. Similarly, it would be inappropriate with something like a growth share / hurdle share.
In the case of minority interests of normal shares in a trading company, dividend yield is often used as a way of cross checking the main valuation method. So you might find the PE ratio of a suitable listed comparator company, work out the undiscounted value per shares and then discount it by, say, 65% to give £100 per share. Then you’d use a dividend yield method to get a value of, say, £80 per share as a way of cross checking. When you decide what dividend to use in the calculation, you’d normally guess at the expected value of future dividends. In some cases that may be the same as historic dividends. But just using what was paid last year without justification is not appropriate.
As you hint, your client’s estate will be in a tricky position if the gift is made today (we expect dividends of £1 per share so the shares are worth £10) but when HMRC come to look at it after death, they see that a dividend of £100 was paid the day after the trust got the shares and so, using hindsight, HMRC will say that they were worth massively more than £10 when they went into trust.
You also mention “class involved” which suggests there may be more than one class of share. That can make things more tricky where (i) the individual transferring the shares holds other shares (e.g. the diminution in the estate may not be the same as the value of the shares that the trust received), (ii) there are associated operations (e.g. give T shares to the trust, S shares to son, D shares to the dog in stages to get bigger minority discounts), and/or (iii) the articles of association are so poorly drafted that the only rights the “T” class of shares have is to a dividend if the directors, in their infinite wisdom, decide to pay one to the “T” shareholders (in which case the shares may be pretty worthless and dividend yield is inappropriate - but exercising the discretion to take value out of the directors’ own shares and pay it on the shares held by the trust may be a CLT). My extensive experience of looking at articles of companies with funny classes of shares (classes A to Z, for example) is that some are drafted so that the rights of the shares are certain and support their value. And others? Well, I have seen more robust teapots made from milk, sugar, and cocoa “mass”. Valuing shares with no rights can sometimes be particularly easy (they are worthless to a hypothetical purchaser).
I found that valuation specialists were great at valuing shares and getting their value agreed with HMRC. But they often did not understand what the particular tax requirements were around the valuations (e.g. diminution in value) or the commercial background of the transaction (e.g. tell the a valuer that the client expects a high value because of x, y and z reasons and they will just stare blankly at you because it makes no sense to them). So it would be worth explicitly telling them what you need them to value, especially if their business has up to now just been valuing shares for EMI schemes.