I fly back to Kuala Lumpur annually from the UK, and usually pay between 520-550£ for flights.
There are direct flights out from London, but I prefer taking Etihad which has a layover at AbuDhabi because it breaks up the long flight a little and the luggage allowance for Economy class is the highest at 30kg.
Malaysia is very multi-cultural, as GB has previously ruled Malaysia, you will see a lot of British influence in language and architecture, and this is probably most prominent in Penang. Also once ruled by Portugal, history has left its mark especially in Melaka. Highly recommend visting these 2 places. Suggest Melaka as a day trip (around 1.5hr ride from Kuala Lumpur), and Penang as a 2day1night (4hrs from Kuala Lumpur).
Kuala Lumpur is the capital, it’s congested, busy, crowded. But it’s worth visiting for the museums and galleries, there are also tourist spots dotted around the goverment official areas such as Butterfly Farm etc. Shopping in KL is great, for high end shopping there’s Starhill & Pavilion (Bottega Venetta, Prada, LV etc). Pavilion also has a load of high street brands, but you will notice british names (Topshop and the likes) are inflated here due to high tax. Pavilion does have a great selection of restaurants, with the more fancy ones on the top floor, and wallet-friendly ones at the food court in the basement.
If you are a foodie, strongly recommend that you try Japanese food on top of local cuisines because it is so much cheaper than the ones you can get in the UK. My favorite ones are Kampachi in Pavillion and Ozeki in Menara TA One.
English is widely spoken and understood in the city areas such as KL, Penang, Melaka.
As for islands, Langkawi is probably the most commercialized island there is. It’s a big island with plenty of activities (eagle watching etc). I have not been to Pulau Redang but have heard amazing things from friends. Your other choices as PP mentioned are Tioman and Perhentian. Between the 2, Perhentian is certainly less commercialized and the water here is incredibly clear for snorkelling. If you are a diver though, you would head to Sipadan in East Malaysia. Not sure when you say Borneo, do you mean East Malaysia or Indonesia? Borneo is a really old term, nowadays otherwise known as East Malaysia (consists of Sabah & Sarawak) which is part of Malaysia. Some people also term Indonesia as Borneo, but that’s another country altogether.
Cameron Highlands is worth going for the amazing scenary, you will be awed by the tea plantations. It’s a 2.5hr drive from KL.
As your trip is pretty long (4-5 weeks?), you can even venture out to Fraser Hill.
Budget wise, it really depends on the type of accomodation you prefer. For options such as Traders, Mariott, Westin which could go up to RM400 per night (80£). Food is very very affordable in Malaysia as it’s really a ‘live to eat’ country and eating out culture is much bigger than the UK. I would say 15£ per day for 2 person goes a long way in terms of food. Public transportation in Malaysia isn’t the best, within KL you can reach popular places by the LRT (tram / underground), buses are hopeless so I don’t suggest attempting it. Taxi is easy to flag on the roads in KL, but do beware that cab drivers there are not neccesarily as ethical as ones in the UK / America. They try to charge you a flat rate, which is usually a huge rip-off. Always ensure they have the taxi metre turned on. You could rent a car for RM80 per day (16£), PM me if you need the hire company contact, I can get that off a friend. Petrol is about 5 times cheaper than in the UK, YES you read that right. Malaysia government subsidizes petrol, so that could be your choice when traveling outside of KL. Within KL, traffic is crazy, taxis and LRT is recommended.
Hopefully this isn’t information overload. Always happy to know someone wants to travel to my home country! Don’t hesitate to ask if you need anymore clarifications!