StaceyUK

59 minutes ago, g33k said:

That's really the new BRP vs the prior (BGB edition) of BRP, tho.  Not a CoC-specific comparo (other than the one "Sanity" bullet-point).

Nick's own summary -- "BRP is a massive pick-and-mix buffet you can use to roll your own RPG" -- is really spot-on.
Don't think of the BRP:UGE rulebook as "a RPG" that's ready-to-play.  I like the "toolkit" simile, and "build-your-own-BRP;" but the "buffet" idea works well, too.  It's a compendium of BRP rules-variants, with which you can build any RPG from a no-magic / gritty-street-rats, to pulpy-Indiana-Jones, to 4-Color-Supers, to Tolkienesque high-fantasy.

Rivers of London is based upon the novels of that name by Ben Aaronovitch.  The RPG was designed & written with the specific idea that it would appeal to fans of the novels who may never have played any RPG previously, so it leans-in on simplified mechanics (and adds a bespoke magic-system, to better match the novels).
It's still BRP.

As for the CoC "... character percentile stuff from Call of Cthulu" this is most-likely that CoC7 switched the "basic characteristics" (STR, DEX, CON, etc) from the original 3d6 scale to a percentile score like the Skills are.  This makes the "Characteristic x 5" roll (which is mostly the only at-the-table roll) much more straightforward.

Each specific BRP-based game (such as RuneQuest) has its own variations & bespoke subsystems; but they are all minor variations that work smoothly with the core BRP engine they all share.  There's no reason you couldn't or shouldn't take CoC's %Stat over to RQ, or RoL, or your own FrankenBRP game.
 

(edit:  are we being helpful, at all?  If not, can you say something further, to help us help you?)

This really helps to answer my questions. Thank you.

Thank you for this!

I'm looking to buy the Basic Roleplaying game engine but I've read reviews that state simplifications from Rivers of London are missing. I'm new to Chaosium games so I'm unaware of what these are. Also some character percentile stuff from Call of Cthulu.

What are the rule differences between the three?