There is no 'cheap alternative' currently. My experience is probably well beyond it's sell by date but Brittany only give discounts to very regular travelers. Hope someone will be along to contradict me though!
We get discount with Brittany Ferries (I think its only about 10 or 15%) on the Santander run by quoting a friend's Brittany Ferries Membership number. You can do this as it asks on the form whether you are a member or a guest. So, if you know anyone who does have membership, ask them if you can use it. I think they get a perk every time you use it so its not asking too much. We looked into the membership ourselves but you would need to make more crossings than we do to make it worthwhile.
We now only do one crossing a year and find it worthwhile, even in the first year with the joining fee it is possible to cover the cost with one return crossing. Add in the other perks, breakfast allowance etc, £10 every time a friend uses your memb. number and it is still viable. If you have a relative or close friend that uses BF they can become an associate member for an extra £30 or thereabouts and obtain the full 30% discount and the other perks.