In terms of Professional Indemnity Insurance, this is a very significant and ongoing cost for the smaller practice, much more expensive than the statutory Public Liability and Employers liability Insurances.
Its as Thomas says, many large organisations will set a minimum level, often plucked out of the air and unrealistic. When challenged they will often accept a lower figure.
Whilst a small practice can easily obtain PI cover of £2 million, for any job requiring more than that most Insurers will only consider offering cover on a case by case basis. This can be prohibitively expensive, especially as the premium is ongoing and cover lapses when you stop paying the premium. Rather than go down that path you could set out the level cover you feel able to offer the client and give them a copy of the insurance certificate with the contract for the avoidance of doubt.
If you are a member of a trade association they may be able to help find cost effective insurance.