I bought an IWC from MW T5 a few years back. With the weak Sterling it turned out extremely beneficial to me.
To be honest you'll be very lucky to get a better price in the UK. If USD is weak you may get lucky in the US (but prices are adjusted often to keep the price level globally), and the higher priced watch, the more likely a discount can be bargained. Some dealers or retailers will give good discounts if you pay cash.