When you look to get a quote for your business insurance, your cover will be in amounts or limits of indemnity.
For example, you buildings, contents, stock and computers will be in amounts that it would cost to replace. Whereas your employers, products and public liability will be in limits of indemnity, the maximum payout the insurers will make, either any one claim or in total in a twelve month period.
Professional indemnity insurance is a liability insurance cover, you do not get a physical asset insured, but a maximum limit of indemnity. The question you will need to ask, is what is the correct limit. You can still get a limit of indemnity of £100,000 if you look carefully enough. The big multi-national corporations and banks will have limits in the tens of millions and after the last few years, boy do they need it.
You on the other hand, may only need to have the cover in place as a protection for your business. The advice or services you provide, could potentially cause a business to have a financial loss. Think worst case scenario, if one of your contracts went horribly wrong and everything you recommended went wrong, what is the most that you could have to pay out? The £100,000 limit may seem high, but in all likelihood you will need to think about limits nearer to £250 or £500k and upwards. The premium differential is not huge, so it may be worth considering a higher limit.
On the other hand, if you are working for a company that insists on you having this cover, then you will need to have in place a limit at the amount they choose. There is really not that much choice here, if they say a £1,000,000, then to get the contract you need to have the cover in place.