A more affordable alternative would be the Wilton Way of Cake Decorating series, better known as the Wilton Encyclopedias. There are three books to the set, with pattern books for each. One of these (I forget which, and mine are in a safe deposit box), touches on Lambeth method.
These books are more readily available from eBay, but the pattern books are rare. The rarest of the books is Volume 2, as there were less printed of this book than of the other two in the series.
The patterns are available through the pattern locator on the Wilton website.