My personal opinion is to not have a reserve when I am certain that a piece has good value,
but I understand you wanting to preserve your investment so a reserve price of £1,500 would be
a good place to be.
Insurance/replacement value is what you had asked for and I'm sure you understand that
it is higher than what an item's actual retail would be. The painting at auction would bring
£1500 to £2,500 based on sales of his other work.
I would suggest Sotheby's London,
34-35 New Bond Street, London England as a first choice for a good auction house.
They can be nose-in-the-air and do turn down many good items, so be prepared for that
and to move on to another if they say 'not interested'. They have sold his work in the past, so
I hoping they will accept your piece with no qualms.