To answer my own question, yes, you could.
https://www.uktradeinfo.com/Statistics/Pages/TaxAndDutybulletins.aspx Total alcohol revenue: £10,219m (£10.2bn)
http://www.ifs.org.uk/bns/bn09.pdf Total alcohol revenue: £10.6bn, total tobacco revenue: £9.8bn
http://www.nta.nhs.uk/uploads/alcoholcommentary2013final.pdf
http://www.ash.org.uk/files/documents/ASH_694.pdf Total cost of smoking: £2.7bn
http://news.bbc.co.uk/1/hi/health/8086142.stm Perhaps as high as £5bn
So yeah, there's no real valid argument against legalisation from a purely financial standpoint, seeing as the tax revenue generated far outweighs the total cost to the NHS.