I don't usually delve into the code of conduct for ISPs as I've never gone through this process from what I can remember, so bear with me.
If TalkTalk have promised a minimum sync speed (since you joined or renewed before 1st March 2019) of 62 Mbps, then according to their code of conduct you need to inform them that your speed has dropped below this guarantee and give them 30 working days (yes, "working" days... a while in reality) to resolve the issue. Since the contract has been renewed in November 2018, you must check the minimum guranteed line speed since it may have changed since when first contracted - unless the speed you have specified is from their November 2018 renewal.
If it is the case that they cannot aid you in improving your speed above their minimum guarantee then you should be able to walk free without early termination fees. But I must stress that it is important that you retain and have at hand all the information regarding guarantees they have given you should you need to fight your case if things do not go smoothly. They probably would have emailed you when you renewed with those details, or perhaps it is visible in your account - the reason why I said it is best to have those details in writing is in case the estimates on your account have updated after renewal (I'm not saying they would try and pull a sneaky on you, but you can never be sure).
If you want to make a complaint, then you can do so by phone, live chat, post or email as mentioned
here. Just make a note of all the dates and times contacted. It may be better to do it via email or post, just so you have a contact trail with all the times and dates listed but that's just my opinion. It's completely your choice to do what is convenient for you.
Just bear in mind that you must do all you can to rule out issues with your equipment and wiring. Some steps here which TalkTalk has listed
here on their website is a start, though I know not all are relevant - it's just important to do what they have specified prior to starting a complaint since they may ask you to complete them anyway. Ruling out internal issues is also another good idea since if they have to send out an engineer and your hardware is to blame, then you may be charged for the visit. TalkTalk may want to send you out a replacement router, which tends to be quite common for ISPs to do when a complaint is made about speed (they just want to ensure that their supplied device is not faulty).
This part is quite important: I am not sure about it, but the new
Ofcom Broadband Speeds Codes of Practice from 1st March 2019 onwards specifies that the right to exit applies to bundled products "such as landline services on the same line, or pay-TV services purchased at the same time as the broadband service". HOWEVER, since you renewed before this date, be aware that this may not be the case. You may wish to confirm with TalkTalk since I really have no clue - perhaps someone else will know.
The way I see it, they would be breaching their code of conduct if they did not help you if you made a complaint or prevented you from leaving without penalty after 30 working days specified in their code of conduct if they have not resolved the issue.
As for your question regarding Plusnet, I've never had issues with their network back in the day. I remember they moved from BT WBMC Shared to Dedicated some years ago, and I never noticed any change since everything was still running smoothly. Just be aware that if your modem is syncing at 50-55 Mbps, changing provider should not make any difference to that since the Dynamic Line Management will probably find that is the best speed-stability balance - unless the package you take is on a wholesale product which is capped under the speed you currently sync at then it would be less.