Arrivals lounges close 2pm, but start packing up around 12pm (e.g. breakfast, massage appointments). You can't use the Terraces unless you stay (i.e. Transit) airside. Transiting, shower, massage all airside then trying to exit and go landside then get to London for anything meaningful then come back is probably a bit much, even in 8 hours. I would suggest you try the Arrivals lounge for a shower if you can then go in to town...or skip all that, go straight to London then head back a little earlier than you would otherwise to go to the Terraces before your DXB flight and freshen up there.

Also, see the ULTIMATE guide on the front page for more on lounges/access/offerings, etc.