At present, all but non-essential travel is prohibited. To travel internationally from the UK (and the return), you have to have a valid reason to travel. Unfortunately, a pressing need to start conducting business face-to-face does not count as valid at present.
There are certain jobs which qualify for travel – Coronavirus (COVID-19): jobs that qualify for travel exemptions and the full list of those jobs is on the UK Government website. It includes everyone from aerospace engineers and airline pilots and crew through to water supplies and sewerage services workers.
Travelling abroad during "Stay in UK" restrictions
So, to emphasise this once again, whilst the “Stay in UK” restrictions are in place, you are only allowed to leave the UK from England if you have a reasonable excuse. It is illegal to travel abroad without a reasonable excuse, and travel abroad for holidays is not permitted.
If you have a reasonable excuse, then you must first fill in the Travel Declaration Form.
Remember to complete this form in good time before you travel (at least 48 hours before departure). Also, you should print off this form so you have physical copy of it. You may be required to show it at your port of departure, or on your way to your port of departure. The website also says “You may carry evidence to support your reason for travel,” which probably means it’s a good idea to have that evidence with you, just in case.
Before travel to the UK
You must also complete a passenger locator form, which must be completed online any time in the 48 hours before you arrive in the UK.
You’ll need to show your form when you check in to travel or board your plane, train or ferry to the UK.
On arrival, you must quarantine for 10 days, and take a coronavirus test on day 2 and day 8 of your quarantine
You have to take these tests and quarantine, even though you provided a negative test result to enter England.