The IFS report does not present the data for 2017. It is not an annual statistical bulletin, it is an economic model using linked and pooled administrative data.
Overall, graduates from RG universities earn more. That doesn't mean that they all earn more than any graduate from a non RG university or that there are not differences between the average salaries achieved by graduates from different subjects and different universities. It is an overall trend.
For economics particularly, it is also a trend driven by a few universities whose graduate salaries bring up the average for the RG group as a whole. The fact that economics graduates from LSE earn on average a zillion pounds, confers no special status on the graduates of Liverpool.
For economics graduates, broadly the trends can be explained by a process of cumulative advantage.
On average, economics students at RG universities (or generally higher ranked ones) enter HE with higher A Level grades (including A Levels in maths), they also have various other background characteristics that have been shown to correlate with successful graduate outcomes. This can be seen in some RG universities more than others.
Once at university, they gain further advantage due to the content of their courses and due to the targeting of certain universities by the highest paying and most prestigious employers.
The main high paying areas where economics graduates have an advantage over graduates generally are those that involve econometrics and other high level mathematical modelling and finance and in particular financial modelling. Generally speaking the highest ranked universities have the most mathematical economics courses, so their graduates are the most likely to have the necessary skills for these kinds of jobs. This is not exclusively the case. There are some universities that specialise in financial economics but are not very maths heavy and their graduates on average command higher salaries than those from universities that are considered generally better for economics as a whole because their average salaries are brought down by people choosing to go into lower paid areas of economics.
The prestige and targeting by employers is an additional layer and helps even those with a less mathematical focus. On average, the High Flyers Top 100 Employers target just 24 of over 140 universities. For investment banks and consultancy firms where a lot of the highest earning economics graduates are found, this figure is just 14. For top banking and finance employers generally, it is 21. These employers invest a disproportionate amount of their recruitment resources in attracting graduates from their target universities, offering all kinds of opportunities and events to build their brand amongst students from the universities they think highly of and recruit the most students from.
But this is largely irrelevant to the question of whether Loughborough is preferable to (what I assume is) Northumbria for economics, even just based on salaries.
It is actually very difficult to compare the two because Northumbria (or Sunderland and Teesside) do not have enough economics graduates to appear in the LEO linked career earnings data. I assume it must be a relatively recently established course. Either that or it is actually designated as a business course rather than an economics course.
Based on the LEO data (which is quite old and statistically unreliable due to the small numbers involved), in 2017 Loughborough economics graduates who were five years post graduation had a median salary of £35,000 (low high range £28,700 to £43,900). This put it at 24th out of 60 in the rankings of economics graduates by median salary. It's not in the most elite group where median salaries are around £50K, but it's good. There is no data for any university that would match the criteria of non-RG university in the North East.
The best comparison that could include a NE university is looking at business, rather than economics. For business graduates the figures were:
Loughborough - Median £41,400 (£31,600 - £51,600) 6th / 121
Northumbria - Median £25,600 (£19,300 - £34,100) 59th
Sunderland - Median £23,600 (£18,100 - £30,600) 84th
Teesside - Median £22,100 (£17,300 - £27,990) 102nd
The numbers have to be taken with an enormous pinch of salt as they are based on the salaries of between 170 and 505 graduates from a single graduating year.
So in terms of salary Loughborough does have quite an advantage and it keeps that advantage even when you control for various background characteristics like parental occupation and some prior attainment.
Whether such salary figures constitute a good reason to move universities is a different matter again. If you care very much about getting the very highest salary you possibly can given the choices available, then it would appear that going to Loughborough might give you a bit of help in this. If you care about anything else, such as university experience, the type of job or employer you want, working conditions and hours, social value, and so on, meh, very few economists starve.