Hi Dave,
You'll have no problem with bike registration if you have original docs with you. They all like to see international vehicle registration docs but it's the original they want. As you are English they may want an international driving licence - see the AA
I just rode London, Bratislava, Lviv to Kiev, then down through Romania and Bulgarian coast roads to Istanbul, Izmir and back to Kiev.
Central and western Ukraine roads are terrible right now - lots of repairs going on and the corrupt traffic cops at every bend.
From Minsk the road from Kiev to Odessa is fine and pretty quick.
The final stretch of Ukrainian road to the border to Galati is terrible - about 10 kilometres of ripped up ashphalt.
The border crosssing at Galati is fine but a bit complicated; you have Ukraine, Moldova and Romanian borders linked up over 1 km. Before your enter the Moldovan section buy green card insurance. It only costs 25 hryvna , - they will accept euros. If you don't get it the Moldovans will send you back to buy it ... it will cost you another hour. BTW ride to the front of the queue - ignore trucks and cars. There are no bribes or dodgy payments - I can't recall if you have to pay an entry visa. You DO have to pay for one to get in to Turkey (about £30) and you have to buy green card insurance if you're insurance doesn't extend to Turkey.
BTW The bigger petrol stations will accept credit cards. OMV is good - they do food as well as petrol - and they have air at each pump too
Going to somewhere like OMV saves yiou time as you get petrol food and air in one stop.
If you time it properly you can leave Romania and clear Bulgaria in a day.
The coast roads through Romania and Bulgaria are mainly ok - I went Constanta, Varna, Burgas, Kirklarelli. (On my return from Izmir I went north through Bucharest - bad decision. All the interior roads in Bulgaria and Romania are in bad condition)
Bulgarian mountains are amazing but there is only one petrol station between the last town and Turkish border - about 100 miles - so make sure you keep your tank full.
Accomodation in border towns is expensive, there is some camping on the coast.
Turkish roads are made in heaven but Istanbul is terrifying - stay well clear of buses.
Going through Russia into Georgia is not on - see other posts about the border on HU - and the ferry route to Istanbul is haphazard - according to friends in Istanbul it operates when it feels like it. Also, you'll need a visa to get into Russial for a Brit in Belarus that will be a tad complicated and expensive.
I'll be in Kiev in Mid september preparing to return to UK via Italy. Call me if you are around.
Enjoy your journey
Jake