No, you can't do that, period.
The fibre network (FTTP) doesn't go via cabinets. It goes via fibre aggregation nodes. The cabinet will be a "leaf" on this network, i.e. there will be a fibre cable from the cabinet to the nearest fibre aggregation node, but you cannot spur another fibre connection off the cabinet. So whatever the farmer may or may not have laid, it's not what you think it is. It might even be a business leased line, which is completely separate from the FTTP network.
If you're only 50 metres from the cabinet, then presumably you are getting the full 80/20 FTTC already? So you're looking to go Ultrafast?
What does the BT Wholesale Broadband Checker say when you check the farmer's address? Look for either "WBC FTTP" (which means they do have FTTP) or "FTTP on Demand" (which means they *don't* have FTTP)
The main option available to you is to order "FTTP on demand" via a business ISP. The cost of this will depend very much on where the fibre aggregation node and/or splitter is. If it happens to be within 500m of your property (which could be true if the farmer does have FTTP), then
new trial pricing means you may pay £1625+VAT to £2650+VAT installation. If that doesn't apply, then under the normal pricing expect to pay £8000+VAT or more. They *might* be able to use the farmer's existing trenching for part of the route if it's suitable, and that may reduce the cost a little, but don't hold your breath.
Just to find out whether this works or not will cost you £250+VAT up-front (non-refundable). It'll take a month or two for the survey and then you'll get final confirmed pricing, which has to be paid in full within 30 days. It could then take from a few months to a year or more to install. Plus you'll have to pay a
higher monthly fee for the first year of service.
The other option, if there are multiple properties near you that want to club together, is an Openreach Community Fibre Partnership (CFP). This has no up-front fee, but can take 6 months or more just to get the survey done.