OK, so I'm going to assume £60K per year of spending for 3 years starting 3 years from now. The basic idea is to create a "conveyor belt" from Singapore dollars to British pounds. So I think I'd do something like this....
1. Start converting S$60K per year (=S$5K per month, or S$15K per quarter if you prefer) now into British pounds. Keep spare Singapore dollars in decent, laddered vehicles such as SSBs, T-bills, fixed deposits, and short tenor Singapore Government Securities.
At this pace all S$300K will be converted within 5 years, just in time for the final £60K. Since you're Singapore dollar cost averaging into British pounds that'll help, and your Singapore dollars are working on the Singapore side in the meantime.
2. On the receiving end it looks like it's awkward to buy U.K. gilts (government bonds/bills) directly, but that's one possible option. Bearing in mind there's some interest rate/principal risk (but probably not huge), how about one or a couple British pound bond funds? Maybe (London stock exchange symbol) IGLS and/or INXG?
Interactive Brokers could do all this except perhaps for the Singapore dollar parking (which you can already do).