What does the は particle relate to?
It is attached to イギリスで, so that is the context for everything that comes after it.
Why 2 particles が?
These are two sentences, one is a relative clause (i.e. a subsentence that qualifies a noun). The subsentence is
今までよりうつる力 が 強くなった
and it it qualifying 新しいコロナウイルス.
The second GA denotes that this 新しいコロナウイルス is the subject of the overall sentence.
What about this より?
Like you said, より describes a comparison. The bit before より loses out with respect to what comes after より。To break this down a bit:
今まで - until now
より - the “until now” loses, so it is “more than until now”
うつる力 が - the power of being contagious
強くなった - this is a combination of 強い (in adverbial form) and なる - to become - in past tense, so all together it is “became strong”
Putting this all together:
イギリスでは、- in England, (the following happened)
新しいコロナウイルスが - a new Coronavirus
今までよりうつる力が強くなった - that has a contagiousness that became stronger than until now
広がっています - is spreading.
(in English relative clauses usually come after the relating noun, that’s why I turned that bit around.)
Does this make things a bit clearer? If there are specific parts you don’t understand, please ask again and I will try to provide some links or the like to explain them further.