With the 19 December 2020 COVID 19 measures, can I travel between the UK and the Netherlands?

NB: the situation is changing so rapidly that this answer will go out of date in a matter of weeks, if not days.

For UK residents, there are two aspects to this: domestic restrictions and foreign travel bans. The British government currently restricts foreign travel for those who live in Tier 4 areas. Here's a handy map of where the Tiers are currently:

If you're in Tier 4, you're not allowed to leave home except for a few specific reasons.

• work, where you cannot work from home
• accessing education and for caring responsibilities
• visiting those in your support bubble - or your childcare bubble for childcare
• visiting hospital, GP and other medical appointments or visits where you have had an accident or are concerned about your health
• buying goods or services from premises that are open in Tier 4 areas, including essential retail, but these should be within your local area wherever possible
• outdoor recreation or exercise. This should be done locally wherever possible, but you can travel a short distance within your Tier 4 area to do so if necessary (for example, to access an open space)
• attending the care and exercise of an animal, or veterinary services

As far as international travel is concerned, only travel for work, for medical treatment or for caring responsibilities likely applies, so casual tourism is out of the question for Tier 4. However if you live in Tiers 1, 2 or 3, international travel is unrestricted and you are even allowed to travel to an airport located in Tier 4 in order to board a plane:

If you live outside a Tier 4 area you may still transit into or through a Tier 4 area to travel abroad if you need to, but you should carefully consider whether you need to do so. In addition, you should follow the public health advice in the country you’re visiting.

Now let's go over international rules. According to IATA's special COVID page, the following rules are in effect for the Netherlands:

Passengers are not allowed to enter.

• This does not apply to:
• nationals and residents of the Netherlands if arriving from a country other than the United Kingdom;
  ...
• British nationals if arriving from a country other than the United Kingdom;

So you could fly to the Netherlands tomorrow if you first make a stop in a different country, on two separate legs. For example, you could do it via Turkey which has no restrictions whatsoever on British nationals. First, book a flight to Istanbul:

Then, book a separate flight from Istanbul to Amsterdam. You will be subject to a 10 day quarantine on arrival.

In the other direction (Netherlands -> UK) there aren't any particular restrictions, although you do need to self-isolate for 10 days or up until you take a COVID test. As always, be mindful of being a potential carrier and ideally get a COVID test before you leave.

TL;DR: its legally possible, unless you live in a Tier 4 area (mostly around London, Kent (Dover and Folkestone channel ports) as of today).

Hull (ferry to Rotterdam) is in a tier 3 area.

Harwich (ferry to Hook/Hoek of/van Holland) is in Tier 2
(ferry train starts at Liverpool station, London in a tier 4 area).

The US is not banning flights from the UK. So, you can fly from the Netherlands to Britain via the US. There are then the usual rules for quarantining that need to be followed, but it should be possible to reach the UK from the Netherlands this way.