I think they still do a deal where you can buy a silver renewal for £150 - so probably not worth spending more than that. But if you let it lapse it takes a couple of months to get the transaction through, during which time you may find you've lost the status credit for your upcoming flights. You have three options - (a) phone/e-mail and beg; (b) hope they use discretion and keep you at silver without you asking; or (c) earn the miles by doing a there and back in one day.