The Saudis can go to Turkey for a joint investigation, but the Russians can't go to England for a joint investigation. It looks like Erdogan is more interested in doing things properly than is PM May.
Not only that, but the Russians actually have a legal right to a joint investigation. The Saudis don't have quite that standing under international law.
Why is the US putting pressure on the Saudis but not the Brits? Why does the US simply take the Brits' word for it while not trusting the Saudis?
It's not as if the Brits are buying as many billions in US weapons. In fact, the Brits actually compete against the US. What could it be?
How about neocon Russophobia? Well, the neocons aren't really phobic about Russians and Russia. The neocons simply want what the Russians have: Russia.