Can I still get from Heathrow to London on a double-decker bus?

I feel I must take issue with dan's answer where it is stated:

There is now no way to get a single bus from Heathrow airport all the way to the center of London.

This is true but only during the day. London has an extensive network of Night Buses which, as the name suggests, run through the night. The vehicles used for these routes are the same red buses as for day routes.

You might well hear that night buses have a poor reputation for passenger safety - however, while this may have been the case in the past, these days with CCTV on pretty much every bus, things are a lot better.

Now, specifically to your point, there is a night bus N9 which runs between Heathrow Airport and Aldwych, close to Trafalgar Square. You can get the timetable from the TfL website, but to summarise:

• The journey takes around 75 minutes
• Buses run about every 20 minutes from around midnight to around 5am
• The journey isn't particularly scenic outside the centre, as London is after all mostly suburbs
• Depending on the time of year, much or all of your journey will be in darkness. Which might not be a deal-breaker for you; after all, cities are lit up at night...

There is now no way to get a single bus from Heathrow airport all the way to the center of London. Most places will require at least three changes. You can search for bus routes on the tfl website (the official London transport website): http://journeyplanner.tfl.gov.uk if, under "I prefer these modes" on the right hand side, you untick all boxes that aren't "Bus".

National Express from Heathrow Airport are long-distance coach services.

They do run services to London's Victoria Coach station from Heathrow, but all National Express coaches are single-decker.

The image you are seeing of a double-decker bus at the bottom of the National Express coach website is a link to their partner company, National Express Buses, which runs local bus services in the Midlands. You couldn't use that vehicle for a Heathrow to Victoria journey, which is a coach rather than a bus.

National Express coaches are high-floor so you're at a higher position than the lower-level of a double-decker, or a normal single-decker, but not as high as the top floor of a double-decker. They generally have rather large window pillars and the windows are all curtained (and the curtains, even when fully open, can block the view) so they're far from ideal as viewing platforms.

[Aside, in British English, a coach is intended for longer-distance journeys, with few, if any, intermediate stops and is therefore likely to use faster highway-type roads; they usually have better-quality seats with more room and belts, have under-floor storage for luggage, and may have a toilet; buses are intended for short-distance journeys with many stops and tends to stick to city streets and the resulting lower speeds; they usually have more basic seats with no belts and have grab bars for standing passengers, have luggage in the main passenger compartment, certainly don't have a toilet, and often have multiple doors to reduce dwell times at stops]