Singapore Airlines is currently promoting promotional fares between Hong Kong and a variety of European cities starting. The prices are highly competitive, especially for Singapore’s excellent offering which we reviewed here.
One highly attractive element to the promotion is that one-way fares are also discounted. For example, Hong Kong to London via Singapore is available for around $2,300 with wide availability in coming months.
Remember that as you transit Changi you should also be eligible for S$20 of vouchers to spend there, as we explained here.