Op l am a LL and l would also be very unhappy that my tenant had done major works without my approval....
Your version of quality may be very different to your LL, also surely your tenancy agreement states no changes to be made without permission.
Most LL are happy to allow a redecorate if the 1) approve the paint and the professional contractor to do a good job.
Over the years, l have had a tenant who redecorated at a cost of 5K to herself (!) without telling me, when l inspected, the rooms were mostly pastel shades of pink ....although professionally done, when she left, l had to fork out to have them redecorated in neutral shades again..resulting in having the flat empty for an extra 2 weeks.
.Had she asked me, l would have happily come to an agreement on colour, and even split the cost with her.
Most tenants ask, if they are planning on being there long term and we agree...ie one set of tenants were going to stay 3 years, and wanted to put up lots of large artworks, as the property is victorian and has lathe and plaster walls this is very difficult and not a diy job, it is stated in the tenancy that there are to be no fixings without my consent for this reason.
But l recognise it is a home for my tenants, and therefore we agreed l would pay for a professional to put in the relevant fixings for their art, ( so that l could control any potential damage) and they would pay for a professional to make good when they left....that is how it should be done.
I put in bathrooms that are supremely practical as well as look nice, ie very expensive shower fixings, as the cartridges last 10 years as opposed to 2 or 3, and it is a difficult and disruptive job to replace them...i doubt you would have even known this....I use porcelain tiles so that they will not chip and can withstand the often harsh chemicals some people use on them, whereas, ceramic will chip after a couple of years and stain badly. Would you have the experience to know if the plumber was good or bad? an unseen leak can cost thousands...
My bathrooms probably cost triple the average, but are incredibly robust and practical, avoiding void periods and cross tenants when things break....hopefully now you understand why your ll may not be delighted and feel you have done him/her a favour.
I hope it all works out fine, but, why oh why, do tenants not just read and follow their tenancy agreement, it is there to protect BOTH parties...