I generally tend to take my dog to Finland if I am going for longer than a week, otherwise it's not worth the 'hassle'. My brother adores Nano so he is always happy to dog sit when we go on holiday.
Travelling with a dog under 8kg is relatively easy in the EU. The Brits make this slightly more challenging as they have some of their own rules and regulations. First of all, your dog needs to be microchipped and have a pet passport. You can get both from your vet so if you plan on travelling with your dog, ask for a passport when they are doing the microchipping. Your dog will also need all its usual vaccinations upto date and a rabies vaccination. You can travel 21 days after the rabies shot has been given.
You are allowed to fly out of the UK with a dog under 8kg in the cabin, the problem arises when returning as the UK does not allow dogs to fly into the country with animas inside the cabin, don't ask why. Not all airlines accept pets so it's worth having a read on their websites. The dog needs to be booked in over the phone and there can only be 2 dogs per flight. Having said that, I have never not gotten Nano a ticket so it's unlikely that your booking won't be accepted. As booking flights over the phone is more expensive than online the best way to book is to book your ticket on a site that has 24h cancellation policy, once you have your ticket, call the airline and ask for a dog ticket. Normally they will take your booking and then confirm the ticket a few hours later. In the unlikely case that the flight already has two dogs, you still have the option of cancelling your flight. The price for the dog ranges between £30 and £50. We normally fly Finnair from London to Helsinki. Your dog needs an appropriate carrier and airlines have size regulations for these so check before you travel. The dog's carrier should fit under the seat so a soft one is better than a hard one.
As you are not allowed to fly in to the UK with the dog in the cabin, you have to either drive from Paris or take a ferry across from Holland. For us the ferry option is way more convenient and have done it lots and lots of times. The ferry is very easy to book from the Stenaline website and again, you need to book a place for the dog. Before you travel your dog needs to be wormed by the vet and needs a stamp of proof on its passport. The worming needs to be done 24-72h before getting on the ferry, they are very strict about this so do not try and cut corners. There are two Stena Line ferries crossing from Hook of Holland to Harwich per day, one at 2pm and the other one at 10pm. The day ferry is slightly faster and works out a little bit cheaper as you need to pay for a cabin on the overnight ferry. We chose the overnight ferry as it worked with our flight times and it's actually quite nice to get a bit of sleep onboard. We flew with Nano in the cabin from Helsinki to Amsterdam with Air Baltic, you then take a train from the airport to the port, Hoek van Holland Haven where the boat leaves from. The train journey is 1h to 1h 15 minutes depending on whether you have to change or not. The check in for the Ferry closes 45 minutes before departure so it's worth keeping that in mind when planning your flights.
On the ferry you have to take your dog to the kennels, they are very clean and well maintained. I've had times when Nano has been the only dog in and times when there are 10 others. If you have booked a cabin, you will be able to watch your dog on doggie cam to see how they are doing. There is also a dog walking deck incase you want to take your dog out. You are allowed to visit as much as you want but the dog might find it stressful with you going in and out.
The Stenaline ferries are quite nice and have two restaurants, a couple of bars, some casino tables, a shop and even a cinema. If you are on the night ferry it's quite nice to go for dinner and maybe a drink and then retreat to your cabin for a read and a sleep. The restaurant on the ferry was surprisingly good and both of us really enjoyed our meal. I had a lobster soup to start, he had parma ham and melon. We both had the very delicious and perfectly medium rare fillet steak for main and desert was crumble for him and creme brule for me. The boyfriend even played a couple of rounds of roulette and managed to double his money, with my help of course.
When you arrive in Harwich you go through passport control and then hop on a train straight to Liverpool street, it takes about an hour and a half.
We left Helsinki on a 3pm flight and arrived at home 9am the following morning. It's a long journey but a relatively comfortable one, neither of us nor the dog mind doing it. I am how ever keeping my fingers crossed that the UK might one day bring their regulations in line with the rest of the EU and let dogs fly in or for Eurostar to allow pets on their trains.
I hope this was helpful and more than happy to answer any questions or clarify if anything was unclear.