House Share Fixed Term

Hello Boardsies

I am currently living in a houseshare:

12 month contract until October (funnily enough no term is stated on the actual lease, but it was likely said in an email somewhere)(4 months left)
3 persons co signed the contract
1 person wants to move out and is searching for a replacement (he has another place rented)
It is proving difficult to find a replacement

I understand it is the person moving out's responsibility to find a replacement; however, in a hypothetical scenario where they just move out and don't pay their share of the rent (agreed what each persons share is separate to the contract; and paid in one payment to landlord) for the remaining four months, who is liable for this amount?

i.e. if this person doesn't pay, what can the landlord do, if say only 2/3rds of rent is paid? could the landlord pursue all co signees or just the one who hasn't paid? I'm not suggesting we would do this, just trying to understand implications, as the remaining two may not be able to cover the cost of the third person's portion of the rent and may be in a predicament if the person moving out leaves us 'high and dry'.

Any other advices would be welcome.

I'm not sure if the fact the lease doesn't actually state a term, assist in terminating the lease earlier, as we wouldn't want to renew in any event, and it may suit us to move sooner.

As an aside, i don't see the property as registered on the RTB website.