I notice that the highest level of the campaign is 14th level. What if each gold sovereign collected as treasure was worth an experience point and the experience point table was set up like this:
Level : xp to attain
1st : 0
2nd : 1000
3rd : 2000
4th : 4000
5th : 8000
6th: 16,000
7th : 32,000
8th : 64,000
9th : 125,000
10th : 250,000
11th : 500,000
12th : 1,000,000
13th : 2,000,000
14th : 4,000,000
15th : 8,000,000
16th : 16,000,000
17th : 32,000,000
18th : 64,000,000
19th : 125,000,000
20th : 250,000,000
This leads to some very powerful characters, not because they obtain high levels, but mainly to attain these high levels, they need to accumulate vast fortunes, and at high levels, most experience gets accumulated through treasure collected. If the wealth level is to high, one could set an experience point equal to 1 copper penny's value of treasure collected instead, the doubling experience point table will still keep the levels down low.
With this kind of advancement, players are more likely to become political actors in this campaign world, they are likely to have their own private armies, mass combat will get used a lot at the higher levels. Players will get to direct troop movements, and one can dust off Battlesystems to resolve these actions. If your player characters want to challenge King Richard III for the throne of England, this is the way to do it.