Are the coupons free travel and you have to book them in advance?
Using a fip card, is there any advantage of booking in advance or just as good using at time of travel?
FIP coupons are indeed free travel. I don't know though if and how they're seen by the UK tax authorities (in some countries, something is added to your taxable income for FIP coupons you received, so they're not really free). Also, if you've just started to work for the railways (often less than one year ago), you may not yet qualify for them. There's usually a limited number of FIP coupons you're entitled to each year so if you plan to do more travel in Belgium and/or the Netherlands, you should check how many coupons per year you're entitled to for these countries. Brussels - Eindhoven with FIP discount will not be very expensive (see below).
A FIP card normally doesn't give any advantage of booking in advance, although there are some quota-controlled services like Eurostar or Thalys, where seats for FIP passengers can sell out. For normal tickets, you just get a 50 or 75% discount on the full fare (like in Belgium and the Netherlands). Although in Italy, for long-distance trains, the cheapest public advance fares can be lower than a FIP fare, even when using a FIP coupon. Also, the 75% discount cannot always be given when you buy a ticket on the spot; I don't know if that's possible in Belgium.
Public fares for Brussels - Eindhoven via Breda (single fare in €, from
NSI website; NMBS/SNCB is always 1 € more):
Full fare: 21.20 + 15.60 = 36.80
Advance fare (7 days, Mon-Thu): 10.50 + 7.50 = 18.00
Advance fare (7 days, Fri-Sun): 14.00 + 10.00 = 24.00
FIP single fare (75% discount in .be, 25% in .nl): 5.30 + 7.80 = 13.10
The RDG has a lot of documentation
here. Especially the section
Staff travel in Europe, with the document "Travel tips for Europe" will be of interest to you. It has a general section and a section per country.