if you use 'last summer' or any other way of implying WHEN something in the past has happened (last night, two days ago, when i was a child), you have to use the PAST SIMPLE not the PRESENT PERFECT
yes, 'have been' is used for actions that are still happening, (one of its usages) but it can also be used for actions that happened in the past and are finished but we dont know 'when' they happened or we are not interested in it
e.g I have been to London
says you have visited London at some point in the past..can be last month, two years ago, ten years ago...doesnt matter...what matters is the action itself, not WHEN it happened
so in your case the correct one is "I went to London last summer"
hope it helped
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