If I were you, I'd put Birthday & TIHWD at 700k and Starstrukk at 1.5M since its been 5 years since the last update.
+
UK sales
One of the Boys era
I Kissed a Girl - 730k (700k without streaming according to the most recent update)
Hot n Cold - 740k ( Its probably around 710k without streaming)
Thinking of You - 90k
Waking Up In Vegas - 190k
Teenage Dream era
California Gurls (featuring Snoop Dogg) - 810k
Teenage Dream - 590k
Firework - 1,05m
E.T. (featuring Kanye West) - 500k
Last Friday Night (TGIF) - 460k
The One That Got Away - 350k
Part of Me – 370k
Wide Awake - 350k
Prism era
Roar - 1,1m
Unconditionally - 130k
Dark Horse (featuring Juicy J) - 930k (? not sure)
Birthday - 160k
This Is How We Do - 100k
Feat
Starstrukk - 605k
If We Ever Meet Again - 435k
I took the sales from last year and I updated them with a few thousands. She should be around 8.650m from her singles and at 9.7m with the collabs which makes her one of the best selling single female artists in the UK.
Teenage Dream ~1,300,000 (1,200,000 as of March 2014 and it re-entered the chart multiple times)
One Of The Boys ~710,000 (661,397 as of March 2012)
PRISM 410,000
The PRISM era was so ups and downs Roar and DH are two of her biggest hits and have tons of sales all over the world whereas the other 3 singles are basically nothing
It hasn't been certified on the Aria page yet I don't think, to Prismatic's question. TD has passed 350k in sales so it should be certified 5 x Platinum
---
Also, if we're gonna use estimations didn't we get that TD figure from MediaTraffic in February or something saying 6.5M? I think a fair estimation for current sales is 6.6 or even 6.7M
WOA should have saved the sales 
For Australia