This plan is crazy. Why on earth would you spend probably over £1000 on a pair of GTX 1070s (nevermind the rest of the system, that £ being only for the GPUs) only to later upgrade them to 1080s, which are currently available and so you could just buy anyway today. A laptop 1080 would set you back ~£1000, so a pair of them would cost about as much as an ultra high end laptop would on its own. Also by the time you did upgrade to 1080 SLI, these cards would be out of date, having been replaced with a new gen of GPUs, and so may be both harder to source and fundamentally worse value for money.
Coming back to the question, I think your best bet would be to stick to a single 1080, and then buy a new laptop when you want to upgrade. Which you will easily be able to afford, thanks to the huge fortune you've saved yourself from not buying 1070 SLI which you then replaced with 1080 SLI).