Be very very careful of companies that offer services like this , some in fact many just clean up a stock engine from a breaker and sell it on, the industry is rife with it unfortunately.
That one posted "Auto Engines UK Ltd" is one of them . ANY company that will only accept cash should be avoided as you have zero protection if anything goes wrong.
If you really want a rebuild the best option is to use a specialist , its not cheap but many people find out its good value AFTER trying alternative means
I would never deal with cash for something like this, it screams scam and i wont be going near that one