I wish to purchase a 40% share in a business in Uk, that has cash flow
and is expanding rapidly into other countries.  I need to raise 12
million pound. What
is the best way to do this.

You need to hire a qualified financial advisor for an investment of
this magnitude.  I can't offer you financial advice, but I can make
some suggestions that the Researcher or your Financial Advisor can
follow up on.

As you are buying a minority stake, I assume you can't do a leveraged
buy-out secured on the assets of the business.  However, you can
consider borrowing through a vehicle that borrows with claim to the
future dividend cashflows and/or equity as security.  You will likely
be expected to contribute a significant portion of your own equity,
though.  In addition, the quality of the cashflows and the credit of
the company is important.

An alternative is to seek a private equity contribution, but this
might be difficult, unless you somehow add valuable strategic or
management value to the investment and there is an IPO story down the
road.

You might also consider tax planning in your investment structuring.

you should in any case consult a financial advisor. 

the 'best way' depends on what your goal is, as well as whether the
package give you some or full control over the enterprise.