With most firms I would say that £20k would be worth offering, as you are also saving both IP and creditors form 3 more years of messing around with monthly installments, which cost in administration time and resources. CreditFix, however, are known to be keen on the full outstanding payments being offered, although do tend to still find success with lower offers once persuaded to present the offer to creditors ( smaller offers mean smaller fees, which are often on a percentage basis).
I see that you are adding in the customary 12 months in lieu of equity release --- I assume that you have looked at values and are confident that there is a high probability that the equity release clauses would actually be triggered, giving rise to the extra payments.
I am not qualified to give advice and can only state my opinions, based on my IVA experiences.