How best to spend £150k on digital marketing

PPC advertising on search will get you the quickest return but it doesn't build for the future. So it's a good quick fix, but it all really still really depends on the state of your site and marketing setup. You want:

1) A smooth email marketing operation, to be able to send out newsletters, special offers etc
2) A great e-commerce enabled website
3) The ability to make landing pages for your ads

* without 3), PPC won't be nearly as efficient as it could be. without 2), even if you do drive traffic and punters to your site, if they can't buy there you're again not as efficient as you could be.

SEO is ok, but it's just 75% backlink building which you can do internally just as well as any SEO. Get a good quality SEO to audit your site (£2k or so), then make sure that you implement the suggestions in your redo and have them audit the redo as well once you're done. With 150k, you're better off hiring someone to do content writing (landing pages) and get good quality backlinks for 30k. That leaves 120K for a website redo and email marketing setup if you don't have all the above, and for adwords and affiliate stuff if you do.