DNS Question

I have taken over a site and the DNS is a bit of a mess right now.

1. Domain Name is with UK2.net who is also a hosting provider however the site is NOT hosted with them.
2. Website is hosted on a VPS with a company called 1-grid which is CentOs 7, WHM, cPanel, PHP etc. Lets pretend the IP Address of the VPS is 50.50.50.50 for example.

So seems alright so far, but I need to switch email to Google for Business, which is normally pretty simple, but here is where things get a little strange.

The DNS Editor on UK2.net is not just pointing the A Record at 1-grids VPS to 50.50.50.50, in fact the A record on UK2.net is lets say 100.100.100.100 which is no server I know of at all in this process.
However the 2 NS records are pointing to the 2 name server addresses at 1-grid.

So my question is, to add/edit the MX records I need for Google For Business mail must I do that on the Zone Editor in UK2.net or the Zone Editor at 1-grid. I assume if the NS records are pointed at 1-grid then all the DNS is handed off to 1-grid so the alteration should only happen at 1-grid and nothing changes at UK2.net

I have no idea why if they wanted all DNS handed off to the VPS they did not just do a Domain Name transfer, or if they only wanted the VPS to do the website portion they did not just hand off the A record.
I have no idea which server, both with full zone editor records I should be touching now.

Please help, anyone?