Thanks for your question, I am Sam and I am one of the UK tax experts
1.On shares ,is there any distinction in the UK between short term gains ( less than a year ) and longer ? I'm sure there used to be , but I've been abroad for 35 years . Only the 30 day rule, which changed to stop the same day purchase and sell to realise a gain - which use to be more commonly known as bed and breakfast!
2. If I sell a watch (inherited abroad 25 years ago - no value attributed ) for £50,000 , is there any tax due ? Yes, but the first £6000 is exempt, as this is deemed to be a personal chattel. Then capital gains are due on the balance.
2a) If so, can I offset this by realising shares at a loss ? Yes - the loss would be applied first, then the annual exemption allowance (currently £10,900 and to increase to £11,100 after 06/04/2014) then any remaining gain would be liable to capital gains tax.
However, if you sell, and are still resident abroad, (and would have been for at least 5 years, during which the sale takes place) then as long as you sell before the tax year that you return to the UK, then you avoid UK capital gains - but then would have to consider the capital gain position in your country of current residence.