I am slowly getting my points balance up with my velocity points and I am aiming to book a return to London. Now as an ‘example’ I can see the taxes on the way there and back and hugely different. Is that just different fares or will it be always different?
It is worth redeeming a flight through Velocity to London or transferring to Krisflyer Miles and booking through them since the flight is operated by Singapore Airlines anyway?
Any help would be appreciated 
There are a few guides on the site that will give you the answers you need
http://www.pointhacks.com.au/krisflyer-vs-velocity-taxes-comparison/
The taxes you pay for your award bookings are made up of a few different taxes; some charge fuel surcharges, different airports charge airport specific taxes to fund their maintenance, departure taxes, arrival taxes and more.
It’s not uncommon for outgoing passengers to b subject to different taxes than inbound, and I know the UK has a pretty massive passenger levy (not sure if it applies to inbound or outbound, but it’s definitely only one direction that it applies to) that makes their airports amongst the most expensive in the world.
If you’re using the same airline and cabin class in both directions the fare will be the same so the taxes will be largely the same, except for whatever differences in taxes that apply. So yes the difference in taxes can be big, but there’s not really a way to minimize this unfortunately.
Hope that helps!