What if: French invasion of the United Kingdom (1803)

  Possibly but Britain doesn't have a lot of room for a burnt earth retreat and also the country is a lot richer than much of Russia.  IIRC I read the British rejected any burnt earth policy because they decided that, especially in the south, the country was too rich to really deny an invader that much in the way of resources.

  Where it might work, provided that the RN can regroup to defeat the French and isolate their army is that I believe London is already the largest city in Europe - possibly the world - at this point.  As such a huge battle for the city could be like an early Stalingrad, with the French struggling with close order fighting where their organisation and experience is less relevant and the locals not only have numbers but also local knowledge.  This could see the French worn down enough that, coupled with isolation from reinforcement, their army is defeated.  This would be likely to be extremely bloody however.  Mind you there might be some irony in Napoleon dying in a skirmish somewhere in the east end of London.

Did you talk about the Chunnel plan yet? 

  I think you mean 1800 technology.   Might be but I doubt it and also it could be difficult getting surprise as the effort would take years.  Not to mention I think it would need artifical islands in the channel to allow ventilation for the tunnel, which I think might attract attention from the RN.  Even if it was set up I wouldn't like to be a French soldier marching through it reliant on ~1810 technology and also Britain not being able to flood it.  Could lose a hell of a lot of troops that way.  See Channel_Tunnel#Earlier_proposals, for a few more details.  It sounds distinctly less than practical.

  A few rounds of grape shot from fortified batteries set up by the enterence and you could do great slaughter.  Especially since there would probably be very little room to cycle out broken units and bring new ones to the front.

  Not to mention seizing that new island in mid-channel and breaking it up enough to allow some water in.  How are the French going to build this in the 1st place, even without British naval dominance?  Not from the bottom up.  They would need to dump god knows how many tons of material to establish something above the water level and then tunnel down from there to link up with the main tunnel.  Apart from anything else they could still be far enough out that the down tunnel from the island is off the course of the main tunnel across the channel.

  At the time the French started building the tunnel, not at all.  After they discovered the plans - which would be impossible to keep secret - then once they [or at least the ones with a basic knowledge of engineering] stopped laughing, then probably.  Since I can't see it being completed while Napoleon's alive, if at all, I don't think it would be a realistic threat.  The huge amount of work that would have to be done, virtually all by manual labour, even if you could overcome the practical problems of building such a tunnel would probably take at least a decade and probably about as many men as the 1812 Grand Army.

  Very probably.  Think how long and difficult the modern Chunnel was.  Then consider trying to do it from one end only and with ~1800 technology.  Which basically means muscle power of men and horses and possibly some primitive steam engines.  This is without the need to construct the island mid-channel and then tunnel down from that.

  See Thames_Tunnel, for a much simplier project which was started somewhat later and still took 18 years and nearly killed Isembard Kingdom Brunnel, as well as killing many others.

Yeah there are pictures with it clearly, I think it would be primitive and unsafe but could be done.