Technically your throttle is illegal, but also so is your trike. Best way to make your trike and throttle legal is to go through single vehicle approval process. Cost of £55 as far as I remember.
https://www.pedelecs.co.uk/forum/threads/pedelec-law-the-details.37594/
If you are on a budget, check
https://www.pswpower.com/?DIST=RkVFFFw%3D
PSW is my favorite supplier. I am talking about components used to convert bikes, not ready made e-bikes. Personally I find e-bikes available on the market overpriced and not a good value for money.
Woosh would be another...
I like Ananda, but they sell OEM products only as far as I am aware, so not really easy to repair.
Bafang offers great products and good amount of spare parts. I wouldn't call their products cheap.
That is a good thinking... hmmm... Bike is worth £1000, so If I get it, I can always sell it to somebody for £500. This way we will both benefit from this deal. Sharing is caring :)
Oh man! I don't have space for more bikes. Why are you tempting me?
On second thought it would be selfish of me to ask for yet another bike while I have so many. I guess I will step back and just feel sad and left out while everybody else will get a new bike :(
Most budget friendly and best value for money probably would be DIY conversion. Prices of motors and batteries are somehow going down. I have never seen them so low before.
Update
Change of routine. Now 90% of time I use first level of assistance on flattish terrain. Second level is reserved for steep hills. Suspension seatpost changed to rigid one. Comfort saddle to athletic. All pieces falling into places. Bike it is as it should be.
Like TSDZ8 even more
I am looing around for a new bike for my daughter. I can't see anything nice for her, but I have seen many good and cheap bikes for adults on Facebook. I strongly suggest you go for your dream bike.