Both are found. I first thought was is more natural, but I've changed my mind: were is more likely.
On the surface, the logical subject is Several thousand pounds worth, which is uncountable; but in practice, that phrase usually acts as a quantifier, so the effective subject is jewellery, which is uncountable; so either way, the verb should be singular was.
However, in cases like these, there are other considerations.
First, a plural noun somewhere in the phrase (such as several thousand pounds) can influence things, leading to were. (This effect is even stronger if the plural noun is closer to the verb, but that is not the case here).
Finally, even though jewellery is uncountable, we are usually aware that it may consist of a number of separate objects. When there isn't an apparently plural quantifier this has no effect (Some jewellery was found, not were found), but with the several thousand pounds influencing it as well, I think were is more likely.
Edited to change my view of which is more likely.
Best Answer
"Who were these buildings designed by?" Would be correct.
The subject is "buildings", and "were" is the verb.