welcome to our community!
1) please check out these threads here on Wefugees about travelling with blue passport:
Since Great Britain is not a visa-free country listed in the threads linked above, you need a visa to visit GB/London.
2) The refugee council of Niedersachsen states the following:
Every refugee that got a positive Bescheid from BAMF after 01.01.2016 has to stay in the federal state that s/he was allocated to for three years. But there is no obligation to stay in a certain city in Niedersachsen.
It looks like you are free to move from your small town to Hannover.
Source (German language - paragraph "Wohnsitzauflage"): https://www.nds-fluerat.org/leitfaden/10-fluechtlinge-mit-aufenthaltserlaubnis-nach-25-abs-1-oder-abs-2-satz-1-alternative-1-aufenthg/11-2-wohnen-umziehen-und-reisen/
I hope this answer is helpful to you.