I'd think twice about this if I were you.......I'm a commercial landlord of shops and 6 blocks of flats in UK. I'm guessing that supply is for the shared areas (communal) e.g stairwells and therefore only runs hallway lighting (say 3 light bulbs). He, like me, will get 3 monthly invoices that are nearly always for same amount of consumption as these lights will be on timer switches. Could also be the supply for any shared fire alarm if above shops or over 6 flats in block.
It wont take them 2 mins to realise someone tapped into it. So, if you're planning to vacate within 3 months fine, otherwise stick the leccie money in a jar and hope its the landlord asking the q's and not the rozzers.
Would be like me growing in the loft above your flat, taping into your supply and you not noticing the say 20% increase.....except the increase he / they will see will be like 100000%
Worse case is that you are renting from private landlord (Mr & Mrs Jones) who have A N Other managing agent looking after the flat for them. Actual landlord (owner of freehold block, usually developer who built it) gets invoice, posts to managing agent to charge back to Mrs & Mrs Jones to pay their say 1/3 share - the other shares are paid by other owners in the block - that's the first 3 people you hope wont notice this HUMONGOUS and rather sudden increase. Add in the other owners in the block who have to pay their share of the communal charges (aka service charge) and even in a small block that could be 5 people you're trying to blag. The 5 assumes none of the other flats in block are let out. So, say 3 owners who are all used to paying same amount, say £8 each, every 3 months - for say 3 11w light bulbs.
Now run by me what you gonna hang off this install again?? Even a 4th 11w bulb will send cost up by 25%.....they might not notice an alarm clock but growing 'erb? LOL. A 54w propagator light doubles their invoice?? A couple of 600w lights, a few fans etc etc, owners gonna being seeing bills in the higher £xx instead of the £x.
Pay your own electric, a lot less risk chief
Good luck anyhoo if u decide to go for it but bear in mind evidence is obvious and previous invoices could act as records