There is no rule of thumb. £10,000 in a bank account is very different from £10,000 in a pension. Under present rules 25% of the latter can be withdrawn tax free after the age of 55 (soon to go up) and the remaining 75% is taxed as income at whatever the recipient's marginal rate of tax is. Also, of course, there is some element of risk with most pension assets. £10,000 in a pension could become £11,000 or £9,000 at some point in the future because the value is usually linked to stocks and shares in some way. There should, therefore, be no question of trading £10,000 in cash for £10,000 in a pension.
To take account the sort of factors I have mentioned above something like trading £10,000 in cash against, say, £20,000 in a pension is likely to be more appropriate. There is no fixed rule of thumb so without involving an actuary it is usually a matter for negotiation but it should be something like the ball park example above. They should not be treated like for like. They are not.