Hi Bubbles26.
An allotments landlord is entitled to let plots of whatever size it wants, so if previously it had let 10 pole plots and it now lets 5 pole plots that's fine.
However, once let the tenant owns a leasehold interest in the entire indivisible plot and the landlord can't simply declare it to be two plots when once it was one - technically what the tenant owns is the right to the exclusive occupation a defined piece of land, a single indivisible right.
If the landlord wants to double the rent then it will depend on what the tenaqncy agreements says whether this is even permitted, but since Harwood v. Banstead and Reigate BC confirmed by Mullins v. Eastleigh BC it's possible that a 100% increase is unlawful, and the rent review term may also be unlawful under the Unfair Terms in Consumer Contracts Regulation 1999. Can you post the tenancy agreement?
Depending on how the landlord has tried to introduce this rent increase it may potentially have breached the Consumer Protection from Unfair Trading Regulations. Can you post any correspondence you've had from the landlord explaining the rent increase and any demand its made for tenants to sign new tenancies?
As a separate issue if the landlord wants all of its plots to be let in units of 5 poles rather than 10 then it will have to give a minimum of 12 months Notice to Quit and then re-let the old 10 pole plots as two new 5 pole plots.