Please see the following for some tips and info.
400k Velocity can be transferred to 296k Krisflyer.
With Velocity redemptions, it costs 139k in business and 203k in first class one way.
With Krisflyer redemptions, it costs 80750 in business 112625 in first class one way.
With Velocity or Krisflyer redemptions, the point costs for return redemptions are not cheaper than 2 x one ways. However, the only advantage for redeeming return bookings with Krisflyer is a free stopover is allowed. Otherwise, it costs U$100 per passenger per stopover.
The surcharge for Krisflyer redemptions is much higher than Velocity. In general, if you are point ‘poor’, you transfer points to Krisflyer and redeem with Krisflyer. If you are cash ‘poor’ you redeem with Velocity. More details on this in links to PointHack articles below.
If you want to fly Singapore Airlines First Class Suites, this is only available via Krisflyer redemption and not via Velocity.
Flying out of London internationally would attract a high surcharge, so you may consider flying out of say CDG (Paris) or other European airports.