In terms of the market for strategic advice to SMEs (excluding financial services and hi-tech companies), there are a few sources:
* General business advisors (i.e. the businesses' day-to-day accountants). Besides GT, there are other top-20 and regional firms that specialise in working with SMEs and owner-managed companies. You could look them up through rankings in Accountancy Age and similar publications and try some speculative applications.
* Venture capitalists. They often have a significant role, not only in putting up capital, but in using their experience from working with other fast-growing businesses to guide new entrepreneurs. Jobs in VC/PE are notoriously difficult to find and your path in would likely need to be through tapping up personal networks and recommendations. If you have hands-on experience from a successful start-up, this is obviously worth emphasising.
* Business Link, chambers of commerce and regional business networks. These organisations provide a lot of support and advice to smaller organisations and often recommend independent consultants/contractors/small local consultancies to their members. Therefore, one avenue is attend a few network events and try to find out which contractors and advisors are most regularly recommended and highly rated.
The big national/international consultancies that you tend to come across advertising on this site are often too expensive for SMEs, or actively choose not to work with them in order to focus on larger, more profitable projects with larger established businesses. However, there is a lot of job satisfaction to be had from focussing on SMEs, and there's always the possibility that you can have a big impact on the business success stories of the future. Good luck!