I think the plumber would be obliged to register for VAT in the UK, unless they were working as a sub-contractor - if that's the case, the supply can be zero rated.
The place of supply is UK as the work relates to an immovable property - the reverse charge cannot apply even if it is a business to business service because it is a construction operation. There is no minimum turnover registration threshold for work connected with immoveable property. Seems a bit onerous especially if you are only doing one off work.
Check out the Revenue VAT Leaflet on "VAT treatment of services connected with Immovable Property"