Check that it is not classified as fragile item or valuables that are not covered in checked baggage in BA's conditions of carriage, even if you declare the increased value. You also need to check for any exclusions.

It might be a daft question but if it is something that is generally insurable as checked baggage and not one of the items that should not travel in checked baggage, isn't it simply an option to increase your travel insurance cover for value of the baggage, if you are not covered already? You also need to check the exclusions here as well.

In addition you are likely to need to declare it to the customs at the destination etc., but that's an entirely separate subject.

Getting cover for £3,000 for checked baggage is not particularly uncommon. My travel insurance (annual policy) covers €7,500.