Island2Dweller

See my comment above about the possibility of dreadful queues at the tunnels.

"I would've thought that any EU flight to LHR would have to clear UK customs at LHR. I haven't heard of any special circumstances coming from Ireland, but others may know more. " The UK, Ireland and the Crown Dependancy Islands (Jersey/Isle of Man/etc) are in the "Common Travel Area". This allows free travel. In theory, this concession only applies if you are citizen within the CTA, but in practice, UK airports treat all arrivals from Ireland akin to domestics. No immigration check, you'll breeze through

Yes, you sail through the barrier. It's only a few miles from Greenwich (in local government terms, it's within the borough of Greenwich). Once you're underway I'd expect you to pass the barrier in about 15/20 minutes. Although the tide tables are published years in advance, the actual high tide on the day can vary by an hour either way (depending on strength and direction of wind). The Thames pilot will order the departure at the highest point of the tide, so you may be a little either side of the published time of 1830. About a mile after that you sail past the end of the runway for London City airport so remember to duck as the planes come into land....

The GWR website has a habit of refusing credit cards that weren't issued in the UK. If that problem occurs, you can buy the same ticket from any of the other rail company sites. I find LNER easy, but there are plenty to choose from, which will work with non-UK cards.

Any of the train companies can sell tickets for all services. LNER seems to work OK for folk outside the UK.

Nothing new in this, it's been a rule for years. You are unlikely to be stopped at customs - it is a "non event" for 99% of travellers. The customs officers work on intelligence and are generally targetting certain travellers. That said, there are occasional random stops and if you have controlled drugs (especially opiates) without the original containers and prescription then you can be in serious trouble.

Doing Dover to London as a day trip is entirely viable. It's a high frequency commuter service. But rail ticket pricing in the UK is worthy of a doctorate in complication. If you are willing to accept fixed times, then you may get quite cheap tickets called "advance fare" tickets (these go on sale about 10 weeks ahead, thought the release date is not an exact science). If you want flexibility, there is a range of walk-up fares from £31 (only valid on the slower trains, not on the "HS1" service and only valid leaving Dover after 1000) through to £111 (valid on any service at anytime). Those prices are the return. If you want cheap and are prepared to fix your time, book tickets ahead. If you want flexibility, don't buy anything in advance. Instead speak to the staff in the station ticket office when you reach the station and tell them your plan, they will sell you the cheapest option. This is especially relevant because if the "we" is more than three then the staff may have "groupsave" fare options which can knock a third off the walk up fares.

People really do pay with card / phone / watch for anything here, however small the price. Tipping a chambermaid is an unknown concept to most Europeans, myself included. And as for paper bank notes - we don't have any! They are polymer now. Britain has a habit of changing bank notes reasonably frequently, and many travellers have been caught out trying to spend old notes, and discovering that they are no longer valid. So if you want notes as a souvenir that is fine, but be aware they may be superceded.

London is not cashless, but it's getting that way. You can't use cash on buses, and many cafes / restaurants in central London won't take cash either. So long as you have what we call "contactless" and in some places call "wave pay"

The HoHo are fine for getting the "lay of the land", but do not consider them as a transport option in London. I would suggest just do a one day pass, and if there are things you want to return to on the next day, just use normal buses (of which there are thousands and thousands in London) or the underground. Far faster and cheaper.

Beware that getting from Greenwich to Tilbury by road requires use of either the Blackwall Tunnel or the Dartford Tunnel. Both are notorious for dreadful queues. An hour queue for the Dartford in the evening is not unusual. DLR and c2c train will be quicker and more reliable.

Seabourne Venture will be on the Greenwich mooring at that time, which is why Viking won't be able to berth at Greenwich (Info from Port of London cruise ship diary page)

To add to that, it's less than a mile from Waterloo to Trafalgar Square. It is very much central London, albeit on the south side of the Thames

Not especially. About two hours by road - can be more as you have to deal with some of the most congested roads in Europe. You can go by public transport, which will also be about two hours. One change from Piccadilly line to rail at Kings Cross-St Pancras.

If there are still seats available, you can pay the driver and get on. But services can be busy and if the coach is already full with reserved passengers then you will be left behind. If you need to get to the port that day, I would certainly reserve a seat. The trains to Southampton leave from central London (not Heathrow). Not a recommended option if you are starting at Heathrow