Considering collective enfranchisement or individual lease extensions 

The major advantage of an individual lease extension over a collective freehold purchase is speed and independence.  The statutory procedure itself is not necessarily quicker but you are out of the starting blocks much faster in serving a Notice of Claim and commencing the procedure. With a collective claim it is advisable for the participants to enter into a Participation Agreement before serving the Notice of Claim so as to commit them to the process and regulate the basis on which they enter into the joint venture. That in itself can take some time to agree between the participants. In addition, it might be necessary to canvass the opinion of the participants part way through the procedure which may involve putting a resolution to them and putting it to a vote.
With an individual lease extension you are not reliant on other flat owners nor do you need to consult them so you can drive the process at your own pace within the time constraints of the statutory procedure.

A Collective Claim

If management of the building is a landlord’s responsibility then collectively purchasing the freehold will deliver with it control of management, whereas with an individual claim there is still an outside landlord likely to retain some control.
However, that should not necessarily be the motivation behind a collective claim because control of management can now be obtained by exercising the Right to Manage which, although it incurs professional fees, does not incur a purchase price as with a collective purchase.
When the participants’ nominated company has purchased the freehold it can offer the participants new leases, normally for 999 years with no rent. One major advantage of a collective claim is that those new leases can be in a modern form so as to be acceptable to purchasers and lenders. With an individual lease extension there is no automatic right for any defects in the existing lease to be amended. The landlord may agree to remedy any defects but might require the purchase price to be increased to reflect this.

The sharing of professional fees on a collective claim may make that element cheaper than an individual claim but that could be offset by the fact that if some flats in the building do not join in a collective claim, the participants will be paying that part of the purchase price which relates to the non-participating flats.