The basic idea is first to ensure that you get enough return from your 100K to beat inflation which is currently 5%.
Since rental is likely your most major expense, it should be your priority to tackle it. If you are eligible for a HDB, then use the money for HDB down payment will be wise. Else it will probably not be enough for a condo - don't forget you'll still need cash to renovate and buy appliances and furniture. 200K will be more like it, so try saving up to that amount in shortest time while ensuring you don't lose any money due to risky investment.
On the other hand, since you are a Brit, if you intend to go back one day, your 100K may be enough to buy some property in UK? How is the rental yield there?
Other than property, you can also put some money in blue chip STI stocks with > 5% return. I'll leave you to do research there, but it is not difficult to find

.