There's a specific Act to challenge a Will - The Inheritance (Provision for Family and Dependants) Act 1975.
The clue is in the name - is the person not named in the Will a family member and/or dependant on the person who made the Will?
If the deceased had been supporting them in any way they might have a claim - and if they are in dire circumstances such as without home/debt etc they might have a claim.
It used to be that you could get Legal Aid if you challenged a Will using this Act but I think that it no longer qualifies.
If it does qualify still then if you "win" you have to pay costs at "inter partes" rates - which means that you might be getting hours and hours of Solicitor's time at Legal Aid rates but if you win you'll have to pay the full rate which is approx £200 per hour.
I used to work in this Dept and it used to cost about £20-£30K to challenge a Will - if you won the fees then came out of the estate.
So in short - you need to be dependant, and the estate needs to be big enough to cover all the costs.
It's very hard to challenge a Will in this way but we were successful on many occasions.... although usually this was when a couple were co-habiting and not married with children. One of the couple died and no Will meant the house etc went straight to the children so the remaining partner would end up suing their own children to claim back the family home.
You need to give more details to get proper advice - the one work answers aren't really helping.