UK Piano Page Piano Forum
Please read the Piano Forum FAQ for more details. Also, read the piano FAQ for common questions on pianos Please don't ask us to place a value on your piano as an on site inspection is required. Contact you local piano tuner who will be more than happy to help.
I'm afraid that your valuation of £3,500 is the replacement value for insurance purposes and does not in any way reflect the current value of your instrument on the open market.
Any dealer will offer you less than open market value - in the same way that a trade-in value for a used car is less than the garage will sell it on for.
Your options are to accept the dealers offer and solve your space issues or contact your piano tuner and get his advice as to the price for your instrument in your area. Then you will have to sell it privately online or by advertising locally.