UK resident driving Bulgarian company car to UK

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There are only three possible options for a UK resident to drive a non-UK car completely legally:

    If the car is not yours, the registered owner is not a UK resident, and he is sitting with you in the car (in this case he is considered to be the driver)
    If the car belongs to (or to be more precise is registered in the name of) an EU based company, and you are working for that company.
    If the car is on a lease in an other country (for example it's a car hired from France)

The second point is regulated by common EU rules on using non-local, but EU-registered cars abroad. Note that these EU rules also allow students, who are enrolled in UK and are only studying in UK for a set period to use their non-UK car. This is because they can be considered as non-residents.

Excerpts

Driving with a foreign registration number

UK residents aren't allowed to use non-UK registered vehicles on UK roads. The only exceptions are if you:

    work in another European Union (EU) member state and use an EU-registered company car temporarily in the UK
    lease an EU-registered car and use this temporarily in the UK

You can usually use a vehicle displaying non-UK number plates, and not have to tax or register it in the UK, if:

    you're visiting the UK and don't plan to live here
    you only use the vehicle up to 6 months in a 12-month period (1 single visit, or several shorter visits adding up to a 6-month period)
    the vehicle is registered and taxed in its home country

If you're stopped by the police

If your car has non-UK number plates and you're stopped by police, you must show you can use the vehicle in the UK without taxing and registering it here.

You may have to show proof of the length of time you've been in the UK (eg a ferry ticket) or evidence that you and the vehicle are eligible for customs relief.

I was under the impression that there is an international agreement which allows a foreign registered vehicle to be used in another country for a period not exceeding 6 months, after which the vehicle must be taken out, or registered.

In addition to the above, I found this link:

http://www.hughesguides.com/foreignvehicles.pdf

You will, of course require insurance.  The 'Green Card' scheme is valid in the UK and will provide the minimum required cover.