My good friend is an international transactions attorney who lived in London for a few years.
You have to be sponsored by an employer to enter the country for employment purposes.
It is also illegal to set up interviews if you travel there on a normal tourist stamp.
As a result, you must set up interviews from the US and then fly to the interview to have it.
Further, any fabrication facility would hire a local worker above a US Citizen who they would have to process and sponsor at a cost that makes your employment prohibitive. These costs can be above $40k US per year and a condition of the application is a demonstration that you provide a skill that neither a UK Citizen or EU Member-State citizen possesses to fill the job for which the visa was applied for.
The only way around this is to marry an EU Citizen or enter as a student visa and go to school there, or find one of the programs that gives temporary work visas for travel agents and other items and explore the job market for auto fabrication while on a work visa that you can obtain on a temporary basis. These types of work visas may be conditioned upon being a student in the US, and most only last for 9 months but can be extended for up to a year and a half. You should be able to find these services on the web.
www.statravel.com is probably a good start. You could also try a hard core youth travel site like
www.lonelyplanet.com or the expat forums.
Your best option may be to find a job with a US-based motorsports operation and hope that you can progress to a point where you are marketable to a UK corporation or that has offices in or connections to the UK.