Hi! This is indeed a great question and one that merits a lot of thought, as it seems you are putting into it.

I think the big consideration is if you have the technological scale (or can hire/build it) to run multiple sites. There are a lot of companies that started off with one domain and then have acquired their way into other verticals and brands. For example, Zillow in the US operates Zillow.com, Trulia.com, HotPads.com, Streeteasy.com, as well as a number of B2B sites. From that experience, customers don't really care *who* owns the site as long as it meets their needs.

Also, I am assuming that these are all services that you offer, not lead generation sites for others? From looking at wantdesign.co.uk, it seems that way, so that is the assumption that I am going off of.

The pros of having it all under one domain:
1) Easier upkeep (potentially) because everything is consolidated on one platform.
2) Good for SEO in that you won't have to build links to a bunch of different websites.
3) It will be easier to build one brand than four, and will be less confusing for your customers potentially. Also, with all of your services under one brand name, it will be easier to upsell existing clients to your other services.

The cons of having all of these under one domain:
1) You will have less ability to customize the different services according to who the customer is.
2) WantWebsite, WantSEO, WantApp etc are all partial-match domain names which have been shown to be good for SEO, though not necessarily better than brand.com/seo or brand.com/app-design with proper onpage targeting, some links built to it, etc.

In my opinion (as a growth marketer who is also building a brand right now), the best way to go is to find one brand name and build out your services under that. You're too small to even think about being on the scale of someone like Zillow.

Good luck! If you want to discuss via a call, I have spare time right now!