Going by reports, Analysts are expecting Apple Watches to retain 44% of their initial value (based on the Sport) after 1 year and 30% after 2 years. Like any product, the value of the Watch will decrease dramatically initially, so in short, you will likely see a 20-25% hit in value after only a month and this will slow down until it reaches the expected level of 44% after 1 year.
What this means for consumers is that if you are not bothered about 2nd hand, you should be able to pick an Apple Watch Sport up for up to £100 cheaper than RRP after only a month.
Also, if it's got Applecare+ attached, then I expect the value of the Watch to be significantly higher.
I paid £388 for my 42mm Apple Watch White Sport with Applecare+. I've had it for 5 months now, it's in perfect condition with no scratches (so far) on the glass or body. I would expect to get somewhere in the region of £250-£270, only because I have Applecare+, without Applecare+, a reasonable figure would be £220-£240.
Whereas after the year, we are looking at £150 (£170 with Applecare+) and closer to £100 after 2 years.
I think it's a great idea to pickup a 2nd hand AW, try and get one with Applecare+ as well, but you should be able to pick one up for between £60-£100 less than RRP.