You need 600 for silver and 1600 for Gold if based in the UK. If you are resident in Europe or simply have a European address, the requirement is 400 and 800. If you use a European address or are based there then you do not get some of the UK benefits. I am not sure if Amex companion voucher for example is valid, if the credit card is based in the UK but the executive club is based /registered in Europe. Perhaps others will know?
I too have been unable to locate the details of thresholds which seem to have been removed for some reason in the latest update to the club pages. They were generally easy to locate.