Praise be to Allah.
It is permissible for you to not fast on the day you travel, and on the day you return, once you have gone beyond the built-up area of the city.
So if you are travelling from Jeddah at midday for example, you must intend to fast from the night before, and refrain from eating and drinking, until you have passed the built-up area of the city, at which point it becomes permissible for you to break the fast.
The same applies on the day you return, if you travel during the day. You should not break your fast until you have passed beyond the built-up area of the city.
The view that it is permissible to not fast for one who is travelling during the day is the view of Ahmad, and is also the view of ash-Sha‘bi, Ishaaq and Daawood, and it is the most correct view.
The majority of scholars are of the view that the one who is travelling during the day is not allowed to break the fast on that day.
Ibn Qudaamah (may Allah have mercy on him) said, explaining the evidence for the more correct view: That is because of the report narrated by ‘Ubayd ibn Jubayr, who said: I embarked with Abu Basrah al-Ghifaari on a boat from al-Fustat during the month of Ramadan, and it set off, then his lunch was brought to him, and as soon as we passed beyond the houses (built-up area) he called for his meal, then he asked me to join him. I said: Do you not see the houses? Abu Basrah said: Are you turning away from the Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him)? Then he ate. Narrated by Abu Dawood.
Then he said: If this is proven, then it is not permissible for him to break the fast until the houses are behind his back, meaning that he has passed beyond them and is no longer among them.
Al-Hasan said: He may break the fast in his house, if he wishes, on the day he plans to set out. Something similar was narrated from ‘Ataa’. Ibn ‘Abd al-Barr said: The view of al-Hasan is an odd view; no one who is not travelling is allowed to break the fast, according to any opinion or any report. Something different than that was narrated from al-Hasan.
End quote from al-Mughni (3/117).
If the traveller intends to stay in a city for more than four days, then he comes under the same rulings as those who reside there, according to the view of the majority of Maaliki, Shaafa‘i and Hanbali fuqaha’. So he is subject to the same obligations as the residents with regard to fasting and offering the prayers in full.
Ibn Qudaamah (may Allah have mercy on him) said: If the traveller intends to stay in a city for more than twenty-one prayers, he must offer the prayers in full. The most well-known view that was narrated from Ahmad (may Allah have mercy on him) is that the period in which the traveller is obliged to offer the prayers in full with the intention of staying there is more than twenty-one prayers. Narrated by al-Athram, al-Marwadhi and others. And it was narrated from him that if he intends to stay for four days, he should offer the prayers in full, but if he intends to stay for less than that, he may shorten his prayers. This is the view of Maalik, ash-Shaafa‘i and Abu Thawr.
End quote from al-Mughni (2/65).
It says in Fataawa al-Lajnah ad-Daa’imah (8/99): The travel for which the concessions of travel are prescribed is that which is regarded as travel according to custom; it is a distance of approximately eighty kilometers. Whoever travels this distance or more may avail himself of the concessions of travel, such as wiping over his khuffayn (leather socks) for three days and nights, putting the prayers together and shortening prayers, and breaking the fast in Ramadan. But if this traveller intends to stay in a city for more than four days, he may not avail himself of the concessions of travel. If he intends to stay for four days or less, then he may avail himself of the concessions of travel. If a traveller is staying in a city but he does not know when he will finish his business there, and has not determined a specific length for his stay, then he may avail himself of the concessions of travel even if he stays there for a long time. There is no differentiation between travel on land or sea. End quote.
Based on that, so long as you intend to stay in London for eight days, then it is not permissible for you to shorten your prayers or break the fast during this stay. What you have mentioned about hardship or the need to focus and so on does not make it permissible for you to break the fast.
See also the answers to questions no. 132438 and 141646.
And Allah knows best.