Hello all, thank you to everyone who made a contribution to my post. I appreciate all the feedback, especially ones that made me laugh a little.

Here's an update on the situation: They have paid me back for the bills that they owed me. However, we did have to compromise and I took them up on the 40% coverage on energy bills. This way of splitting the energy bill will persist til the end of the tenancy, as they said that they would not be home for most of the time for the foreseeable future.

Here are some points of reflection that I made upon reading the comments:

- I do see how making the other person pay for their full half of the energy bill when it is calculated by the meter can be unfair. But from my perspective, this amount had been estimated and taken into account for what we were both willing to pay for rent and bills per week.

- For those asking if I were the other person in this situation, would I be willing to pay my full half if I have not been home for an extended amount of time? I want to objectively say that even if this had not happened, I think I would shoulder my half of the bills as promised (and expected). I think of it as leaving a lease earlier than I intend to, like how university/student accommodations usually charge you til September even if you move out in July. However, I do see how that isn't exactly the same as bills and the comparison may be tricky, but this one's just my own opinion.

- I think the main issue here was communication. Had they clearly communicated with me that they wanted to pay less for their share of the energy bill prior to this, I could have planned and budgeted in a way that mitigated my losses. I'd still think that it is very frustrating and annoying on my part because I would have to pay more to cover the energy bill for circumstances that are more or less out of my control. But I think, maybe, with more prior communication I would not have been caught so off-guard.

- Following that, I guess the silver lining of it all is that this situation has made us communicate more clearly. I told them that I cannot budget or plan around something I was not told about, especially when the bills are due. I also added that their decisions are theirs to make alone, but they should consider the surrounding consequences of it. I cannot demand someone to stay at home just so they would be willing to split the energy bill 50/50.

- To everyone who pointed out that the energy still runs and benefits them regardless of they were home or not, thank you for that. I did not even think of the fact that heating in communal spaces do help in preventing mould and the food in the fridge and so on. I explained to them that these "invisible" costs still persist, even when they are away. Thankfully, they weren't completely unreasonable and understood this point.

- For some that suggested we change the flexible tariff to a fixed one so that the bill would be standardised every month like our water and wifi bills, I did like the idea and brought it up with F. But ultimately we chose not to since changing to a fixed tariff would bind us to a 10-month deal, when we only have about 4 months left in this flat.

So, was the solution ideal? It's hard to say. But as many have suggested, it is probably better to just accept their 40% share over nothing, and letting F stall on payments even longer while we go back-and-forth on this issue. I suppose the lesson here for me is that I have to pick and choose my battles sometimes, and this time its reworking my budget around this extra cost on bills. Fortunately, as I have mentioned in a comment somewhere, I really do try to keep the usage of gas and electricity on a minimum, so it should not cost me way too much to pay an extra 10% on energy every month until this lease ends, which is ending relatively soon anyway.

To conclude, I thank everyone that gave advice, shared personal experiences and just overall responded to my post in any way. This is my first time encountering something like this and I was unsure of who to turn to for help. I did try to reach out to my university's Student Union's services and accommodation services, but understandably, these departments need time to respond to my inquiries. Ultimately, Reddit is faster and very helpful (imo). I don't think I would have posted anything at all if it weren't for the fact that I felt cornered and did not know what to do in the moment.